Some events are missing on ios caldav calendar

We have about 400 Nextcloud accounts, syncing caldav calendars with Android and iOS devices (and Thunderbird). On 2 Iphones devices of 2 different users, some events are missing. Not all the events, only some. We cannot find a pattern on these particular events. Recreating caldav accounts on the devices doesn’t solve the problem.
After few days, some missing events (not all) are showing up.
By the way, missing events are perfectly visible on Thunderbird and other caldav client on other devices (even other iOS devices).

Maybe a sync token is corrupted on the devices ? How to reset it (delete the account doesn’t work !).
Maybe a particular event is blocking sync ? How can we find it ?

Any help will be appreciated.
Nextcloud 29.0.4
Iphone 13 with iOS 17.X

We have only 5 useres and 10 devices.
But similar topic with two iOS driven devices.

Using NextcloudPi with Nextcloud Hub 8 (29.0.7)
The new occ comand didn’t fix anything.

And yes, maybe ther is a missconfiguration somewhere, but I have no clue where to look at.