A family of Microsoft relational database management systems designed for ease of use.
Copy/Paste is a Windows function, not specific to Access, so let's step back and confirm a few factors.
Does this happen in multiple Access accdbs? Does it happen in other applications?
What version and build of Access do you have installed?
Did the problem appear after a Windows update? After an update to Access/Office?
I assume the Access form or forms where this occurs are not read-only, but please confirm that.
Thanks for providing sufficient information to try to offer suggestions.