Missing GMAIL folders on iPhone - but shows up everywhere else

Ok, this is infuriating. Just recently the GMAIL sub-folders have all gone AWOL on my iPhone 11 ProMax, running the Mail.app on iOS 15.5. On my older iPad Mini 2 running iOS 12.5.5 I was able to add the email account in question -- and the folders all show up properly. Same as my MacbookPro running 10.11.6, and my MacMini running the latest MacOS. I've deleted the Mail.app from the iPhone, did a restart, tried removing and adding back the email account -- nothing. At one point I was able to get the sub-folders to appear, by disabling "show in IMAP" for a parent folder in the GMAIL settings via a web browser. But after disabling the same for the sub-folders (dunno what I was aiming to do by doing so -- if it aint broke don't fix it -- but i fixed it alright, for good) -- I was unable to re-create the "bug" and now I can't see the sub-folders anymore.

Seems like a bad preferences or setting within the Mail.app -- but I don't understand why deleting the app doesn't completely wipe these bad settings/preferences (do they still use that word these days - "preferences" as in the old MacOS 6-9 days). Back in the day you could delete preferences files and the app would re-create them from scratch. This reinitialization process fixed many problems.

It seems that a complete nuclear option -- wipe the darn device is in order. A back-up seems out of the question too -- although I really wonder -- whether the bad corrupt preference files wouldn't be re-downloaded from the Cloud? That would suck.

Or -- maybe -- just MAYBE this is an iOS issue -- a bug that an exhaustive Google search doesn't show other people experiencing -- and it'll be fixed in the next update?

I'd gladly welcome any ideas.