Save and close this file. Check whether its icon is correct, that is, it is preview icon.
Press Command-Up to move one level “up” and then re-visit the folder with your test file. Check whether its preview icon is correct. For me, it often changes to a generic one.
If the icon is correct, edit its contents (for example, add or remove a letter), save it, and then repeat steps 3 and 4. Try this 3 or 4 times.
Rather than looking to see if others can repeat the “problem”, it’s usually more useful to create a new user account on your own system and test there. You can control the features you add back to the user account to isolate an effect that seems buggy. You cannot control anything about any of our systems, and whether we see or do not see the same result, does not validate much about the root cause. Especially things affecting the file system which is highly dependent of possibly dozen or hundreds of local configuration events.
it’s usually more useful to create a new user account on your own system and test there.
Well, this is exactly what I have tried. I have created a new account, but the issue didn’t disappeared. Then I additionally re-installed macOS from on this account, but again, this didn’t helped.
Howefer, if I change the appendix to txt it changes icon.
Well, this is somewhat expected, because your file is still an RTF file, but you force macOS to generate an icon for a plain text one…
It’s not possible to save as .txt from Text edit in Sonoma.
Are you sure? There is no such thing as Menu > Format > Make Plain Text now? (Anyway, Sonoma will be available for end users in October/November, so some bugs are OK for now.)
…and now I noticed something strange!
I saved the Text file to desktop, and then moved it to the same location as the other files. When I put it there, the icon got the same appearance as the text file above. However, when I showed the extension, the icon changed to only showing “TXT”, as per this picture.
I am not able to reproduce this, or get the icon back to the document filled with text!
So, there’s definitely something buggy going on here…
I tested as you described (1-5) and the result was:
no changes when editing and saving to the original folder
icon changes from txt to generic when going to another folder and then back to the original
icon randomly changes back to TXT when editing and saving
icon always changes back to TXT when I uncheck “Hide extension” in the Get Info dialog box"