I’m still using Yosemite and have not taken part in any Apple beta testing. Try trashing the prefs. Start by quitting Lightroom.
Launch Finder and click the Go menu whilst holding down the Option (alt) key. Then go to Library >> Preferences and delete the following file:
com.adobe.Lightroom6.plist (or your version of LR)
Then re-launch Lightroom.
N.B. The Library is hidden until you hold down the Alt key.