This seems to have been fixed upstream last November (by not using dm-tool for locking anymore):
https://github.com/lxde/lxsession/commit/9dfe4035a6b555452046db7d4e430507d7c0e469 -- To UNSUBSCRIBE, email to debian-bugs-rc-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
