It's certainly possible, and I know this very well from considerable personal and aquarist experience, that long-term movement in a different way (in this case erratic and strange swimming behaviour) can continue for a very long time beyond when the original issue might theoretically appear to have gone away / resolved, and recurs especially when not fully conscious/focused purposefully on being able to rectify or carry out the movement/action the 'proper' way. The degree to which it is evident or seemingly absent can also vary too. Aquaristically ( thought I might have invented a word here but it turns out it does exist
), I had an ailing and elderly x-ray tetra who started 'bouncing' and catapulting around the tank, particularly at night when he was asleep, but who was able to control this more during the daytime - I wouldn't discount a swimbladder disorder but it might be that your angelfish can control it better when fully conscious by adapting/moving differently in compensation.
From what you've described, I definitely think he has the best prospect of flourishing if you can continue to keep him in his own tank - sometimes the benefits of that outweigh the theoretical approach of a fish being social and ought not to live on its own. I also had an ailing and elderly harlequin rasbora who I moved into an isolation tank and who went on to a live a further 2 months than would have been the case if I had left him in the main tank where he was clearly struggling. (Like you, I became attached to this fish, in part due to this uniqueness.)
Another example of strange swimming behaviour is of a neon green rasbora with a very curved caudal peduncle area. In his case, the actual cause is unknown - it
might be TB (which some of the other fish possibly were suffering from including many of his shoalmates who died of spine curvatures), it
might be his enjoyment of riding the filter current at an angle which has actually moulded his spine into this curved shape, it
might be a genetic deformity that developed over time.
I'm not sure that you'll ever know a definitive answer to the cause of it, but I hope these examples and experiences help somewhat.