Yes, I can easily replicate this. Thank you! I’ve cropped the video and attached as a GIF to the original report.
It feels that something is wrong with the parsing of the value, thanks to the parenthesis... Ai should ignore it and read the actual value, but I suspect it tries to respect it and skips the whole range of numbers. Very trippy!

Funny thing is — if instead of the arrow buttons clicking you set the cursor in the field and press Up and Down keyboard keys — the bug doesn’t happen. Same goes with hotkeys for leading... And this is how I’ve been changing these forever. Now, when I’m thinking about it — that’s perhaps why. Maybe I discovered it in past and learnt to workaround... Anyway — huge thanks for the help with it.

Looking at preferences> type> size/leading is set at 2pt.

my report was on most recent masos 14.2 and most recent AI - but it has been happening for a long time. So it must be happening both pc and mac. i have a link to video now showing exactly what happens..

edited: https://bit.ly/3wYI6aw - video.

Video shows type that leading works w/ arrow up and down fine, then I change it back to "auto" setting and then when I try to change again the bug happens.

The video didn’t get attached, but I get the idea.
Can you please go to Preferences > Type and check the Size/Leading parameter, just in case, — what value do you have there?

This does still happen, attached a video, but it happens using the arrow icons to change or scrolling....but only when changing it from "Auto" and if you hit less arrow jumps to 1 and more jumps to 1295 .... see attached video.

Yep. It should result the same as hitting physical Up/Down arrow keys.

when I want to change the leading, it's change to 1295 pt or 1 pt.

The same symptoms occur with CC 2019 (v23.0.2) for Mac.

Japanese:
同じ症状はMac版CC 2019（v23.0.2）でも発生します。

Character panel > Leading - when using the scroll wheel to change leading it auto jumps to max amount of 1285pt...very annoying

The occurrence of Illustrator CC Ver. 23 and Ver. 22 under Windows 10 has been confirmed.
(Not occured Ver.21 and less)
The reproduction method is as follows.
1. Create point text in 2 lines
2. Set the line feed setting to "automatic"
3. Select only the first row (It may occur even if all selections are made)
4. Press the line feed spin button
5. Check line feed status and numbers

Screenshot posted on the Japan Forum.
https://forums.adobe.com/thread/2605291

Origjinal (Japanese) :
Windows 10環境の、Illustrator CC Ver.23と、Ver.22での発生を確認しました。
再現方法は下記です。
1. ポイントテキストを2行で作成
2. 行送り設定を「自動」の状態にする
3. 1行目のみを範囲選択する　(すべて選択でも起きる場合はある)
4. 行送りのスピンボタンを押す
5. 行送りの状態と数値を確認する

ジャパンフォーラムにスクリーンショットを掲載しました。
https://forums.adobe.com/thread/2605291

Same here, its really annoying.

Funny enough I'm having the opposite behavior where the leading is generated, not on auto but with over 1200 PT spacing between the lines....

There seems to be a leading bug in the latest version. The leading automatically created is usually too spaced vertically, or creating a text area creates pseudo-Latin lines of text that are on top of each other.