I don't see any forum for bug reports so I'll post these here. Sometimes Poser 12 on the Mac doesn't load libraries the starting up. I have to quit the app and start it up again in order for the ibraries to appear, Also, sometimes Poser 12 hangs when quitting the app, forcing me to force quit it instead. I hope these bugs get fixed in Poser 13.

These issues should be reported here if you haven't  yet:

https://support.posersoftware.com/

FYI Both of these issues are known and the dev team is working on them. The lingering after shut down issue affects Windows and Mac and has proven to be quite elusive. Some task or combination of tasks sets up the shut down issue, the "IT" factor. We've yet to find the actual cause of that one. If any of you figure out the particular combination of tasks to trigger the shut down getting stuck please let me know. I've been hunting that "IT" for months.

I have multiple libraries when I start up Poser. Does this bug affect only those with multiple libraries or does it also affect those with only the default Poser library?

IMO, this has nothing to do with the number of runtimes. I mean; one runtime is only a list with one item. 15 runtimes is the same list, there's just, somewhere, 14 loops more, but the principle remain: from time to time, something is going wrong and the library window remains empty
It can be the database that didn't responded, leading to a unmanaged situation and an empty window for example.

I think that the Dev team should introduce a kind of switch somewhere, false by default. and if set to true, a more complete log file could be filled, helping the dev team to target some deep-hiding bugs, such as the one that makes the RTX Cycles-based engine crash for seemingly difficult-to-find reasons.
I develop all day long in Odoo's framework, a Python-based ERP, and there are some switches that can explode the log file, depending on its setting.