I don't think it every preserved a queue after changing the library.
Well, I recall otherwise.
As a matter of fact I find it
sometimes returns to the Main Library with the Playing Now queue intact, but most times it is cleared of the queue.
The reason I did not post regarding this condition earlier is that I was using MC16 to access the one library and MC17 for the 'Main' library, so was not switching libraries until recently.
Perhaps the "Recent Playing Now" playlist could have an option to retain up to "X-number" of old entries across sessions rather than to be cleared when MC is closed?