This is silly. There is no other way to describe this. Apple you need to fix this.
My wife is very organised and has all her email in folders beautifully sorted.
Then iOS stops this from working reliably!!! And from reading all the messages it seems that you don’t intend to any time soon.
i though that iOS was supposed to be a professional tool. (Very expensive iPad Pro fully spec’ed out) and guess what it can’t do folders in mail.
I have tried all sorts of “fixes” on the web that don’t work but I haven’t found a word from Apple even advising that it’s a known issue and it’s being worked on. This type of attitude just wastes all of our time. Having something that works is what I expect from Apple.
Frankly Apple you need to just fix this.