ios mail app cannot access special-use folders, occurs only on our production cyrus

[Sebastian Pein]

hi there.

i've been following this list for a while, but this is my first posting. 
since there are so many quite experienced people around here, i hope 
someone comes up with an idea. i realize in a way this is not about 
cyrus but more about ios apple mail, but as it only appears with 
mailboxes on our cyrus server, it is. thanks in advance! (my apologies 
if this is offtopic to this list.)

we are running cyrus 3.0.5 on gentoo. it has been updated from 
2.5.something a couple of weeks ago. we are and have been facing an 
issue, where ios mail app clients have difficulties in accessing 
special-use folders. it is a bit hard to describe, because there seems 
to be a change in behaviour over time (that is the time between setting 
up a client, and then using it for say 5 minutes). this happens only to 
apple mail app on ios devices, eg airmail app on ios is fine. the issue 
started with version ~11.1, but i cannot tell exactly, since user 
reports were a bit vague at the time and didnt reach our IT department 
really at one time altogether. it is still occuring up to ios 11.4b6.

so what typically happens, a user comes to our IT department...:

case 1: a user comes to our IT department being unable to delete mails 
from his inbox on his iphone. looking into it we see the trash folder 
appears empty.
case 2: a user comes to our IT department being unable to access his 
sent mail folder. looking into it we see the sent folder appears empty.

both folders of course do contain mails. from every other mail client 
access works just fine. (even mail.app on osx.)

to see what is going on we check the mappings in the account in ios 
system settings. the problem seems to be that mail does not keep the 
special-use mappings, it falls back to the default ones. the same 
happens to the inbox prefix: changing it to "INBOX" and saving the 
changes, will result in the default setting "/" again. but: when 
deleting the account and setting it up from scratch, the settings will 
be correctly saved for a couple of minutes! then after 5 minutes or so, 
inbox prefix and special-use mapping is lost again. (it took me ages to 
find out about the delay!) so since the mapping is not setup right, i 
understand that access to subfolders will then not work correctly. 
affected are only the special-use folders, regular subfolder are 
accessible just fine.

in imapd.conf the "autocreate_*" is set:
autocreate_inbox_folders:       "Sent | Trash | Drafts | Junk"
autocreate_subscribe_folders:   "Sent | Trash | Drafts | Junk"

and since cyrus has been in use for a long time, we are using 
"INBOX"-prefix:
unixhierarchysep:       0
altnamespace:           off

i found postings about similar behaviours, but those were related to 
murder setups. nevertheless i already testet:
suppress_capabilities:        LIST-STATUS
suppress_capabilities:        LIST-EXTENDED
suppress_capabilities:        CREATE-SPECIAL-USE
but that didnt change anything.

also, i removed and had recreated *.sub files where the subscriptions 
are stored, but that didnt change anything either.

why i turn myself to this list? this happens only to our 
production-system of cyrus. i did install a testlab environment, 
identical to our production system, and i was not able to reproduce the 
error. even after coyping over a mailbox from the live system, the 
mailbox seemed to be "cured". our production system has about 4tb of 
mails, so switching is something i would like to prevent.

i really hope the behaviour comes out of an error i configured, so it 
might be corrected with missing parameters in imapd.conf or an operation 
on cyrus configuration files.

does my description make any sense to you? is anyone else experiencing 
this? is there any more precise information i can provide? i am more or 
less out of ideas what to further test. so any idea or hint is greatly 
appreciated.

best regards,
sebastian