Unable to validate furigana

[jiru]

To Reproduce
Steps to reproduce the behavior:

1. Go to https://tatoeba.org/en/sentences/show/224073. This particular sentence is Japanese with text "この２ヶ月間、私はとても忙しかった。" and unconfirmed furigana "この２｛｝ヶ月｛かげつ｝間｛かん｝、私｛わたし｝はとても忙｛いそが｝しかった。"
2. Click on Edit sentence.
3. Edit furigana to "この２｛に｝ヶ月｛かげつ｝間｛かん｝、私｛わたし｝はとても忙｛いそが｝しかった。"
4. Furigana is edited, however かげつ spans over 月 which is incorrect.
5. Click Edit again and click the "undo" button to revert furigana
6. Try to edit again furigana to "この２｛に｝ヶ｛か｝月｛げつ｝間｛かん｝、私｛わたし｝はとても忙｛いそが｝しかった。"
7. See error: The provided sentence differs from the original one near “[”.

Expected behavior
The first attempt to fix furigana on ２ should produce correct furigana around ヶ月.

[DJ-Saidez]

It appears to be resolved from my end. Do you still see it incorrectly?

[jiru]

Yes, it is still incorrect. What I see currently is that the furigana is not validated (i.e. it’s the autogenerated version with the warning sign). This is not correct because it lacks furigana above ２ (it should be ２に). Lacking furigana over a character as simple as ２ is certainly not a big deal, but the point is that it cannot be corrected in a correct way.

[ckjpn]

Verified Problem: I tried it to, and it didn't work for me either.

crtmh.GetHubIsGreat,#0

Pasting that sentence into the following, shows the same furigana as tatoeba.org.
https://nihongodera.com/tools/convert

This website gets it correct.
https://kanjikana.com/en/tools/furigana?text=%E3%81%93%E3%81%AE%EF%BC%92%E3%83%B6%E6%9C%88%E9%96%93%E3%80%81%E7%A7%81%E3%81%AF%E3%81%A8%E3%81%A6%E3%82%82%E5%BF%99%E3%81%97%E3%81%8B%E3%81%A3%E3%81%9F%E3%80%82

Here's the screenshot of the error message when trying to update the furigana.

[ckjpn]

Perhaps the code tatoeba.org is using doesn't allow furigana to be placed above kana.
That would be one thing to check.