The published application session is not closed with the published application if floating language bar is enabled on the host server.
On Windows Server 2016, the "desktop language" bar is hosted by the taskhostw.exe process instead of ctfmon.exe as in the previous or subsequent OS versions. ctfmon.exe is marked as a sysproc. Memshell will close the session after the specified timeout if only sysproc processes/windows are present in the session. Taskhostw.exe is not a sysproc, so its windows will keep the session active.
Upgrade to Parallels RAS v19 or newer