iPhone vs Exchange Active Sync Profile using SMIME

[bofh]

Currently we are having an issue with iPhones.
We cannot enable S/Mime on Exchange Accounts (Active Sync) - while it works with a manually configured Exchange connection.
However it seems to work on an iPad Air.
The Profile is set to enable S/MIME, a certificate is existing.
Setting in Configuration Profile:

Greyed out in Mail Settings:

Profile on iOS -> turns out SMIME is disabled - but why?

Oh and ofc. Latest Version of JSS and iOS.

Does anyone have an Idea how to solve this?

Greets from Germany,
bofh