I don't personally use medibang's cloud save feature but there's a good chance it might be a bug so I'd first recommend maybe not deleting things if you can and second possibly trying to reach out to either a help page or their twitter (they're pretty active there) which is here: https://twitter.com/MediBangPaint_e
I feel like they'd mentioned a maintenance being performed but I'm honestly not sure but the best bet would be to wait a bit before having to delete more art than you're willing