text disappears while "redefining style"

I would try placing the cursor elsewhere in the paragraph and making the change. I'm thinking (vaguely) that multiple changes to the paragraph and style might be in play, and implemented when the style is updated. Check Keep Options for the style; if it has excessively "hard" keep-together settings, turning off hyphenation might make InDesign follow those rules and move the paragraph to a next frame or page (possibly overset or invisible) as a result.

Yes,  I agree with @Robert at ID-Tasker . We need more information to help:

Please tell us what operating system (exactly) you're running. Please tell us (exactly) what InDesign version you're using. Does this happen only with one file or with other files as well?

If it only happens with one file, it could be caused by corruption within the file. Saving as IDML (File > Save As > InDesign CS4 [IDML] and reopening the file can sometimes fix such corruption.

If it happens to more than one file, you may need to restore InDesign preferences or caches. Here is how you could do that:

https://www.rockymountaintraining.com/adobe-indesign-rebuilding-preferences-cache/

Which MacOS version?