I’ve not played around with Flame Blade, though too-easy loss of concentration is a known issue, as is losing benefits from some items (eg Helmet of Grit) when wildshaping then turning back, until the item is unequipped and re-equipped. Could the latter be behind your Circlet of Fire issue?

Moonbeam not being moveable, and Heat Weapon and Call Lightning not being reapplicable, while in wildshape are also known issues, but when I reported these myself a while back I was told it was per design as wildshape prevents spellcasting. I pointed out that this seems to run counter to 5e rules that say “Transforming doesn't break your concentration on a spell you've already cast, however, or prevent you from taking actions that are part of a spell, such as Call Lightning, that you've already cast” and was told this would be raised with devs. That was a couple of months ago, so I’m not sure if the position has changed, but if it bothers you (as it does me, clearly smile) then it may be worth submitting as a bug report via Larian’s support form and seeing what the stance now is.

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"