Connections remain active even after terminating the session in Postgres
If you had permission to kill them with pg_cancel_backend, but doing so did not work, then they must be in a state where they are not responding to interrupts.  That is probably a bug, either in PostgreSQL itself, or in your code, or in pldbgapi.  What does wait_event_type and wait_event say?  Can you attach to them with gdb and get backtraces?