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.)
It turned out to be… Samsung Keyboard in the end, probably reinstalled itself after a system update.
I thought it’d be something like com.samsung.clipboardsaveservice, which was the culprit on my old phone, but it didn’t even exist on my new one.
I’m glad you figured it out!
Samsung is so invasive. I can’t wait until I can justify turning þis þing into e-waste.