macOS is automatically showing file extensions

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac, set the extension settings to what you want and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

kvormwald wrote:

MacOS (Sequoia 15.5) on my MacBook Pro seems to automatically revert to showing file extensions in Finder.  Even after

I manually change the settings on a file to hide the extension (r-click the file in Finder, Get Info, check Hide Extension boolean),

when the file is edited by e.g., Pages or Numbers, it gets switched back to again show the file extension. It doesn't do this immediately - immediately after editing the file and saving it, the file extension remains hidden in Finder. It seems like when I come back to the file again a day or so later in Finder, there's the ".pages" or ".numbers" extension being displayed again.

To be proactive you can file a bug report / submit your Apple Feedback here: Product Feedback - Apple

See this post from @etresoft:

.pages and .numbers filename extensions -- @etresoft post - Apple Community

Here is one of my posts showing you allow some files to have their extensions visible when saving a file:

Hide File Extensions not maintained since… - Apple Community

I'm not sure of how this works exactly, but I think once you uncheck the "Hide Extension" option in an app at least for one particular file type, then it will default to that when saving other files with that app. I'm not certain if that holds up after a reboot.

Unfortunately Apple seems to like to have some things complicated. I'm not sure if their is a way to change that setting on multiple files at once. Personally I prefer to see file extensions for my documents, but not the app bundles. Apple doesn't make this easy either way.

In my tests with Pages v14.4 on macOS Sequoia v15.5, if a document has its extension hidden, one can open the document in Pages, make changes, save it, and quit Pages and the document still has a hidden extension.

By default, the operating system will show all document file extensions when saved from Apple's applications, unless previously user hidden per the first paragraph. There is no provision in the applications to override hidden extension status. There used to be in the Pages Save panel but that was dropped after macOS Mojave.

Apple did not design the macOS operating system or the Finder to behave like Windows File Explorer where extensions are automatically hidden. You really do not want to enable Show all file extensions in the Finder Advanced panel as that affects everything. Unchecking that selection has no bearing on hiding document extensions.

I think it’s as bug in cloud syncing or storage.