Trying to give my photos captions in the Photos app on Mac... it's not behaving...

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.

If the above doesn't work and you have an Intel MacBook Air tru this: Reset NVRAM on your Mac - Apple Support. If that doesn't help try the following (for both Intel & Silicon Macs): Reset the SMC of your Mac - Apple Support

DrYeet wrote: …Is there a fix for this? Why is this happening?

That is certainly not normal. I'm guessing that this doesn't happen in other apps, is that right?

OT's suggestion of trying Safe Mode is right on. That will be telling, even if it doesn't work.

If his other steps don't work, you can also try creating a new user and seeing if the same thing happens, there.

Let us know…