macOS 13.6.7
Illustrator 28.6

Step to reproduce
1. 任意の画像や図形を配置し、その上にテキストを入力する。
2. 画像などとテキストを選択肢クリッピングマスクを適用する。
3. マスクされた形状は実際のテキストが持つアウトライン形状とは異なる。
4. このクリッピングマスクをかけた状態でアウトラインを作成すると表示が変化し正しいマスク状態になる。

期待する結果
フォントの持つアウトライン形状でクリッピングマスク処理される。

実際の結果
実際のアウトライン形状と異なる状態でマスクされる。おそらくフォントの持つBlueValueの影響を受けているのではないかと考えています。

EN
Step to reproduce
1. place any image or object and type text over it
2. Select object and text, apply Clipping Mask.
3. The masked shape is different from the outline shape of the actual text.
4. If you create an outline, the shape will change to the correct masked shape.

Expected Result
The clipping mask is processed with the outline shape of the font.

Actual result
Clipping Mask is performed with a state different from the actual outline shape. We believe that the BlueValue of the font is probably affected.

この問題はバリアブルフォント以外でも再現します。
フォントのデザインにもよりますが、細いウエイトのフォントを小さなサイズで使用したときに顕著です。
悲しいことに、PDFにはこの状態のまま書き出されてしまいます。

既にEgorさんが述べている通り、テキストに「オブジェクトのアウトライン」効果を加えることで回避できます。しかし、そこまで注意して作業できるユーザーばかりではありません。

テキストで作成するクリッピングマスクは楽しい表現ができるテクニックのひとつです。ユーザーが制作作業に集中できるよう、問題が解消することを願っています。

--- English translation (using a translation tool)

This problem is reproduced even with non-variable fonts. Depending on the design of the font, it is more noticeable when thin weight fonts are used at small sizes. Sadly, the PDF will be exported as-is.

As Egor already mentioned, it can be avoided by adding an "Outline Object" effect to the text. However, not all users are that careful.

Clipping masks created with text are fun to use. I hope the problem will be resolved so that users can concentrate on design.

Matthew, yes, I am seeing and am able to reproduce this small dot of 'i' problem.
However, I have a workaround and an explanation of some sort, but they contradict each other in this particular case. Quite peculiar!

A live text in Illustrator (and basically in any other software) is hinted. The generated outlines shifted a bit to fall precisely in the grid of the medium (paper or screen) to avoid blurriness. The more you zoom out, the more the difference between the background text and the clipped yellow blob looks prominent. That's because the clip mask uses the pure unhinted outline to render the content.
If you outline both, they will match exactly, and the black hair padding you see is only because of the stacking, it won't be printed or exported.

To keep the text live, but to get the correct match still, you can apply Effect > Path > Outline Object to both of your texts. This will force Ai to ignore hinting and display pure outlines. The contradiction I mentioned is that usually it is required only for the bottom text and not the clipping group... but in this case they match only when applied to both! Perhaps it’s because this is not a mere font, but a variable one, and is basically sewn dynamically from the pieces.

Sure this has to be studies further. Thanks for bringing this up.

This is somewhat better, yet still not totally fixed. See the highlighted issues with the tiddle of the "i". Also black visible at the edges of the underlying text.

Works fine for me with Hepta and using it as a clip mask.
Matthew, does it work for you now as well?
Please report back if it is fixed (or not).

The effects vanishes if you apply Outline Object effect to a text before clipping.

When I set cap height of a text object to be exactly 10 mm, and then use it a clipping objects to clip an art, the height of the font gets smaller, although Ai still thinks it’s exact.

Please see the file and GIF attached.