Block Error Adding Certain Characters

I’ve been encountering problems with Advanced Text and Advanced Heading blocks when the text content contains certain non-English characters. Recently I’ve noticed the problem with some Turkish letters. For example, create one of those blocks and then copy-paste the following word (the name of a Turkish town) into it:

Ovacık

Note that the letter before the ‘k’ is not an ‘i’.

The blocks will accept this word and it works fine in the front-end. However, do the following:
– Save the post.
– Exit the editor.
– Re-open the post in the editor again.

Result: The block in which you inserted the above word will fail, with a message: “This block contains unexpected or invalid content.”.

Note, using the ‘Recover’ option seems to work 100% of the time and the issue does not seem to affect the front-end. It just requires a ‘recover’ every time the page is opened for editing (and every block such characters are used in has to be individually recovered, which can be a nuisance in some cases).

I’ve encountered the issue with other characters, too, though I did not log them since I didn’t realize at the time that those characters were triggering the issue.

Using the same word in default WordPress blocks (the paragraph block, for example) does not cause the error. It seems specific to GreenShift.