Shortcut to increase font size in Terminal doesn't work in Big Sur

Log into another vanilla admin user account, Set up users, guests, and groups on Mac,  and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).

If the problem goes away in the other account go back to your account and reboot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test 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

If you want the macOS product team to read your request, you need to send them direct feedback. These are user-supported communities and the product teams neither read nor participate here.

That said, I can repeat ⌘+= in the Terminal and each repetition enlarges the font in the Terminal window. A ⌘+- reduces the preceding enlargement, and a ⌘+0 (zero) restores the font size to its original setting in the Terminal Preferences.

macOS 11.2.3.