"Math Symbols" in "Show Emoji and Symbols"

Boot 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 still no math symbols answer this question: have you installed or run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?

[EDIT: I didn't see the prior answer offering this solution before I posted.] Open Emoji & Symbols, then click on the window icon in the upper right corner and that will expand the window to show the Character Viewer in the format you are used to seeing, including the option to choose Math Symbols. For some reason, it appears the default view was changed in Mac OS 11.1.

when i use the "control + command + space bar" keyboard shortcut i'm given the option of choosing the math symbols. are yours not there?

You may have to add it:

you're welcome. i'm glad i was somewhat helpful.