Nextcloud Calendar in iPhone Calendar App / When shared hide this event

Hello all,
i´m not english native, so please excuse if the wording is perhaps not precise enough.
We have recently started using the Nextcloud calendar app. Almost everything works perfectly, except for one strange case: The calendar app of an iPhone, in which Nextcloud Calendar is integrated, only offers “When shared show full event” and “When shared show only busy” in the shared options. The third option that Nextcloud Calendar offers “When shared hide this event” is not offered in the iPhone app.
By the way: When this iPhone Calendar App was connected to a Sogo calendar we used before, these 3 options were taken over by Sogo.
Does anyone have experience with a case like this?
Many thanks!