iOS 17.6.1 Mail app is not displaying mailbox subfolders
I have created Mailbox folders for my email accounts. Each displays beneath the Inbox etc for the account.
I created subfolders for each new mailbox. The mailbox label in the account drop down label has a blue arrow indicating the mailbox contains subfolders.
When the mailbox is opened however the sub folders, (which now contain essential emails), are not visible. The edit button for the mailbox is also greyed out and dead.
The emails in the non-displaying sub folders can be searched for. However they won't open when selected from search.
Restarting the Mail app does not restore the subfolders.
Restarting the iPhone does not restore the subfolders.
Deleting and reinstating the Mail app has been suggested elsewhere but some people experienced significant losses doing this and it seems like drastic action.
Looking through support pages it seems this is a ling standing problem which has never yet been answered satisfactorily.
Is anyone aware of a solution to this catastrophic problem for iOS 17.6.1? If so, please let me know because some of the emails I currently cannot access are irreplaceable and of really vital importance to me and others.
Thank you.
iPhone SE, iOS 17