Save As not working intermittently across programs.

Boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and see if the problem persists. Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 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

Unsigned Files:
  Launchd: ~/Library/LaunchAgents/com.realitymine.roymorgan.community.desktop.watcher.plist
    Executable: ./Users/***/Library/Application Scripts/com.realitymine.roymorgan.community.desktop/uninstall_app.sh
    Details: Executable file is hidden - possibly adware

This appears to be adware. Unless you are familiar with this software and know it is legitimate, you should uninstall.

Run Etrecheck again and follow the instructions to remove it.

  [Not Loaded] com.maintain.PurgeInactiveMemory.plist (Apple - installed 2021-07-11)

You should really not install this sort of thing (presumably, part of "Cocktail", if I remember correctly).

"Purging" inactive memory does not make sense in any OS ever since OS X 10.9 Mavericks.

It decreases performance, often quite significantly, and can cause crashes.

I recommend uninstalling Cocktail altogether.

Give the following a try: boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and see if the problem persists. Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 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

What Safe Mode does is prevent 3rd party apps from launching. Evidently there's some 3rd party app or apps that are interfering with what you're trying to do.

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

It never happens in Safe Mode, does it?

This suggests that something you have installed is playing a role.

Please run Etrecheck and post its full report here.

Use the “additional text” button and paste the report into the text box.