Samsung keyboard is server related features so it is possible that all the stored info is just not updating properly on the server. If the clipboard isn't related to the server then it's possible you just need to clear cache or update the keyboard. I have noticed that the keyboard doesn't really update on-time.
Try to clear cache and see if it fixes, if it doesn't then do a screen recording of you deleting clipboard stuff and check back shortly to notice the copied stuff returning. Follow up with error report submission to Samsung Members. Hopefully the techs at Samsung can figure out what is causing this issue.
1) Turn off your phone
2) Press the sound up and power key that will bring up a menu
3) Press the sound down key to navigate to the clear cache partition item in the list
4) Press power key to select that action
5) After the message appears at the bottom of the screen saying the cache has been wiped, press the power key once more to reboot the phone.
There's something that keeps generating the clipboard risk that Max Security finds and can't clear so I find myself checking it and clearing the cache every day.