Please see image attache. I am viewing a path at 30% opacity and it looks like the below.

Illustrator CS 26.2.1
MacBook Pro Monterey 12.1

Thanks for taking the time to respond so comprehensively, Egor.
Rather than further detract from the OP, I'll start a new thread in a more appropriate channel.

Good questions, Philip and Paul. I am really glad you asked them.

Despite having an Admin badge, I am a user, just like you. I have my own job, real tasks to design, limited time and the desire to make Ai a better tool, because I spend up to 10 hours in it every day, for a majority of my life.

We are biased. There are many combinations of OSs, drivers, GPUs, builds, plugins, settings... and Illustrator is a complex program. Tests and QA staff do catch bugs, but some slip through. The truth is developers don’t know about bugs caught by users until they are reported. Yes, most of the time they don't get reproduced for them.

I am not an Adobe staff member, although I can contact Adobe staff — so everyone else can do. Email, Twitter, LinkedIn, Slack, you choose.
I am just what the badge says — an administrator here, at UserVoice.

My tone can seem harsh, that’s true.
Aren’t you angry because this shit isn’t fixed yet? I am.
This particular bug — I don’t have it. I am on Windows. But I just see how different people report the same issue using different words and phrasing. I know the pain.
I sew these reports together into a larger one, I try to bring attention from the team, I request help from everybody else — because I want to help and expect the same from everybody else. When it’s fixed — then it’s enough, for me.

NFTs suck — yep, this is my private POV. Protect your work, as Paul says.
If you are bound by NDA — sure, don’t upload anything.
But the bug is generic enough to build a simple test file in small time or strip the important parts from the real project and share it. That’s what some users here did — thanks — and yes, the problem really turned out to be Mac-specific.
Will it help to solve this? I don’t know. I hope so.

There are ways to share a project privately, by using sharewithai@adobe.com email, and I forgot to include it in the status update.

And thank you so much for being open about this. This ship has holes all over it, and we don’t talk about this nearly as much as we should.

I think you're right about not uploading .AI files publicly, Philip.
And I'm quite frankly surprised that Admin staff are indicating this is a valid approach, especially as Adobe are a key player in the CAI initiative.
Photoshop has Beta of 'Content Credentials', don't think Illustrator has yet?!
So much work is being stolen for NFTs, my advice is don't give anyone - even Admin staff - access to your source .AI file. Protect your work!

If Adobe reaches out or otherwise provides a place to upload my file directly to them, I can do so. I can't upload my file publicly. I provided screenshots and a description for that reason. Happy to see Adobe taking notice of glitches.

I think you've missed the point, I gave extensive details of the fault, including screenshots and hardware/build info.
This was May 2021.
I couldn't upload the .AI file because it was a live file, covered by NDA and I take my client work very seriously in that regard.
I refer to my earlier point; this bug is clearly manifest on a range of systems reported below, I find it hard to believe that Adobe can't replicate this fault with their own resources, with the data provided below (and respecting both the user's - and clients - data protection rights). And on the subject of time management you refer to, I would suggest that subscribers to this software have more productive uses for their time (e.g. creating work to earn enough to pay for it in the first place) than being seconded to IT support roles which they may be unqualified for (and certainly not recompensed for).

Responding to Admin (Egor) Your tone seems to indicate that users diligently reporting this fault are not doing enough to rectify. Are you saying that you can't replicate this error in-house? Or are you so under-resourced that you genuinely need paying customers to do tech support for you? In what other industry would the vendor be able to pull this magic trick on the customer.

Here is my example. Zoom in to see the artefacts. Screendumps with and without GPU on the second artboard.

In the last updates the problem has worsened and I see it more often than before.

MBP 13 2018, Intel Iris Plus Graphics 655, Monterey 12.2, Illustrator 26.0.3

Applied default gradient to a font.

MBP M1, Monterey 12.2, Illustrator 26.0.3

This is happening to me as well - gradient or solids.

My Setup:
iMac (Retina 5K, 27-inch, Late 2015)
4 GHz Intel Core i7
Memory 16 GB
AMD Radeon R9 M395 2GB

Resolution: 5120 x 2880
Mac OS 12.1

Illustrator 26.0.3

Would be nice to see more focus on bugs, performance issues and optimization instead of new features. These performance issues really limit how someone can utilize Illustrator.

Ten_A さん
コメント感謝いたします。
お恥ずかしながらプレビューに種類があった事を知りませんでした。
処理速度を上げるのは今は難しいので、目をつぶります。
ありがとうございました。

これはGPUプレビュー特有のものでしょう。オーバープリントプレビュー又はCPUでプレビューすると正常に表示されませんか？
GPUプレビューは速度優先で処理するために演算結果が荒い状態になる場合も多々見られます。表示品質を上げていただければ良いんでしょうけど処理速度とトレードオフでしょうからある程度は目をつぶるしか無いのかもしれませんね。

MacOS 15.10.7
IllustratorCC2022

曲線で図形を作成し、不透明度を下げたら実際にはない線のようなものが現れました。ベジェのハンドルを動かすと連動します。
不透明度が100%では見えません。

複製保存で書き出したPDF、スクリーン用で書き出したPNGには表示されなかったので、Illustrator上のプレビュー画面でのみ表示されるものと思われます。

成果物には影響はないのかも知れませんが作業中とても気になってしまうので、修正していただきたいです。

In this video, you can see some broken lines

Radu, which one? You forgot to attach an image or at least give a description. What is broken exactly?

Illustrator v26.0

MacBook Pro (15-inch, 2018)
2.9 GHz 6-Core Intel Core i9
32 GB 2400 MHz DDR4
Radeon Pro 560X 4 GB

I have this bug 3 versions already :(

This is a GPU rendering issue, Ralitsa. Try to disable GPU rendering for now.

In the latest version of Illustrator 25.4.1 the gradient preview appears broken. The same file opened in the lower 2019 version of Illustrator does not have this problem.

GPU issue. Please send the file and your GPU specs to sharewithai@adobe.com
For now you can switch it off and work in CPU mode.