Photos: Search doesn't find items // This Library isn't searchable by spotlight due to its location

I have multiple libraries on external SSDs formatted APFS and have no problem searching within them for file names, objects, keywords, etc.

First try reindexing Spotlight for that particular drive: How to rebuild the Spotlight index on your Mac - Apple Support. It will take a bit for the reindexing.

If that doesn't help give this a try: boot into Safe Mode according to How to use safe mode on your Mac 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.