Grand Arena no battles but score is 2121

Your defense is bugged because CG doesn't prioritize bugs that impact players and our crystals negatively.  That's the hard max score, it's as if you didn't set a defense at all.

It's been bugged like that and impacts players occasionally.  It's only been doing that since Cubs was a content creator here, so I'm sure it'll get fixed any decade now.

I'd recommend putting in a ticket and telling them to give you the crystals you lost out on because of the bug.  Do your one attack to get participation and then submit a ticket with the screen shot.