And it affects their code that they run to validate when you change an extension. Normally if you open a file with an extension already (like say `test.txt`) and try to save it with a new extension (like `test.asm`), it prompts you asking something like "you've entered the extension ".asm" but the standard extension for this file type is ".txt" and gives you the options to change to the original extension or use the one you typed. If you try to change `test.txt` to `test.asm` with this bug, it tells you you've entered the ".as" extension, and not ".asm" like you actually entered.
What you think of as file extensions are merely a matter of convention. Some applications may treat them more seriously, but that should be up to the app to enforce - not the OS.
I can see no reason why a system-wide file dialog should ever be altering this.
That said, I doubt it is something Sublime and others missed in terms of API - TextEdit has the same issue.
For comparison, Xcode actually does register every filetype it supports, so it would be difficult to hit this issue in the official IDE.
Still on Sonoma, or I would answer this question myself.
This is what happens. I'm sorry you can't reproduce it. Are you using mac for this?