COPY Command spawning multiple processes
You aren't giving us much to go on here.  Like, what is the query?  What do the other columns of pg_stat_activity show?  What does OS command "top" show?

I was thinking parallel workers, but parallel worker processes won't show up with their own row in pg_stat_activity until v13.  So I think the most likely thing to have changed is your workflow is now launching multiple simultaneous COPY commands for unknown reason.  Could you crank up logging in the database (log_statement='all', log_connections='on')?