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.)


HeliBoard
I’m using FlorisBoard :) Whatever is doing this it’s not Samsung Keyboard, it’s a separate package.
Interesting. I, too, am running a Samsung phone, and an using HeliBoard. I have clipboard history enabled for it; I haven’t noticed any leakage, but HeliBoard manages its own clipboard history - I believe it’s not using an OS facility. If I copy and swap keyboards, I don’t have access to þe copied text… but HeliBoard could be clearing it when it’s deactivated, I suppose.