A number of people have reported this bug in the official Adobe feedback forum: Lightroom: Find and Full Screen command key shortcut conflict under OS X 10.11. Adobe has formally acknowledged the bug, but there isn't any workaround posted yet. You're the first person to post that it's happened with 5.7.1, so please add your vote and details about your issue to that bug report. Your information could help troubleshoot the bug, and by adding to that topic, you'll be notified when there are any updates.
Thank you for your kind patience.
I would like to inform you that this issue has already been reported to us by many customers.
Our engineering team is investigating this issue.
Thanks. Here's a workaround given to me by an Apple user on their forums in the meantime:
No idea about updates, but I think this one you can fix yourself.
OS X has always allowed the user to add keyboard shortcuts, so I will tell how to associate Command-F to the Find command in Lightroom.
1) System Preferences->Keyboard->Shortcuts
2) Click on "App Shortcuts", then hit the + button at the bottom left
3) In the sheet that appears, select Lightroom for Application, write the menu title *exactly as it appears in Lightroom*, and enter the desired shortcut.
You may confirm that the shortcut is visible in the menu and it works :-