Edit: Samsung Keyboard (in the personal profile) was reinstalled probably after a system update and was the culprit. The issue is now solved. Thank you for your comments.
I have made a work profile using Shelter. I was copy-pasting some stuff in my personal profile while the work profile was disabled. Later, I discovered everything I had copied was showing up in Samsung Keyboard’s clipboard history (in the work profile). Personal profile’s Samsung Keyboard was uninstalled via ADB (among some other packages like Google Play Services). What package could be the culprit? (I’d love to just install LineageOS on it but there isn’t a built for the device yet. I just don’t use it for sensitive stuff.)
I can’t look because I don’t have a Samsung phone, but it’s probably a system app. I’d just look through the list and see if one jumps out.
You might get a better answer on an Android community, because this is really an Android question.