I don't have Poser 11. However, many issues can be solved in previous versions by deleting Poser's preference files, which forces Poser to reconstruct them and to employ other default settings on its next startup. https://support.smithmicro.com/customer/portal/articles/1982174-solving-weird-issues-with-poser-by-removing-the-preferences

Though, it's likely a permissions issue if you see that your Library disappears frequently/always. Win10, especially, has issues with executables that must read/write to external directories in order to function properly. Check to see that Poser has the appropriate permissions. That may require looking for instructions on Microsoft or tech sites. Also note that, in similar cases to one like this, you may have to authorize your username to "own" the directories a program is using and "Run As Admin" rights may not always work properly. But, is should be the case that deleting the prefs files, if there are any, and then making sure that just the executable has the privileges necessary (by right-clicking on the actual executable file and choosing to allow it to always run with Admin priveleges, at least that's the way in other Win versions) should clear most problems.

I don't think Win11 still uses any Flash-derived directory helpers. But, if it does, then expect patches whenever any major Flash updates are forthcoming. (I think they got away from this issue, though.)