The line breaking behavior for Japanese text in the latest versions of the TextMesh Pro package which is version 3.2.0-pre.x is consistent with other text engines such as Microsoft Word.

In Japanese line-breaking rules (as defined by JIS X 4051 and commonly followed by text engines), digits are treated as ordinary characters that can break after or before most other ordinary characters. There’s nothing that makes “10” inseparable from the surrounding kana in your example.

Your text:

これはテストですこれはテストですこれ10テスト

If it needs to wrap near the end, it’s entirely acceptable for the break to occur after “10” so that the line ends with “…これ10” and the next line begins with “テスト”.

…これはテストですこれ10
テスト

There’s no prohibition against having the digits at the line end and the following “テスト” on the next line.

⚠️ What isn’t allowed is breaking within the number (“1|0”) or breaking before the first character of a multi-character punctuation mark, but that’s not the case here.

So yes — per Japanese line-breaking rules, breaking after “10” before “テスト” is permitted.