this is an old bug with xdg-utils, the patch was originally on the bugzilla and migrated but apparently the issue got "lost" and has not been addressed on the upstream https://gitlab.freedesktop.org/xdg/xdg-utils/-/issues/139 i commented the issue to try and get attention to it, but in the meantime it would be great to have this patch merged in debian.
hi dear maintainer, going back to this here's a more proper patch containing the commits from the upstream merge request https://gitlab.freedesktop.org/xdg/xdg-utils/-/merge_requests/118 that altho green lit has no defined turnaround time for merging nor for the next upstream release, if you want a squash of the commits in the patch feel free to answer and i will provide one.
bug is not resolved unless the patch is applied, the behaviour of running xdg- screensaver lock while using xss-lock is still incorrect, ie: it returns this error message: ERROR: Unknown command 'lock' please consider merging the patch from my previous answer to this bug report, or if you want a squash of the upstream merge request please say so and i'll be glad to provide one.