Ok may be a simple one but looks like users on iphones/ipads are having their calendar entries deleted, they have checked on the web portal (SOGO/Roundcube) and they are missing there too.
The iDevice has given power to create, modify and delete events. So if the iDevice (and the user) can, if the iDevice application tells the server “delete the event”, the server comply…
(unless specific different settings. but that’s a whole lot of thinking…)
So.
Did the iDevices deleted the events, as instructed by settings?
Is there a specific location on the apple device that would say to do this (sorry, i knows its out of your remit as you look after the software) but researching it i dont see an option on the device to delete calendar events (obviously if the user is doing it then thats something else)