iOS Element App does not notify with sounds

[LutzLange]

We are using Element iOS legacy App 1.11.3 with ESS 23.08.05

We"re running in a restricted environment.
The app is rolled out to the phones with Workspace One MDM.
On start of the Element App on the phones, a VPN connection is brought up so that the iOS Element app is able to reach the homeserver.

The homeserver is allowed to reach the push gateway through a proxy.

We can make notifications work. They are displayed in the lock screen. We were expecting these notification to play a notification sound as well. But this does not work.

How could we debug this further?
Do we require another network connection to work?

So far, we only opened access from Synapse to https://matrix.org/_matrix/v1/notify

[LutzLange]

We have tested this with 1:1 rooms. But be aware that Admin and Audit Bot joined these rooms as well.

[florian-bellencontre]

I've been trying to solve this problem for a while now. I've tried a number of things, including reinstalling the application.

I've discovered that this bug has been fixed in #7636 and #1809 in version 1.11.1. Nevertheless, the problem is still present for me too, even though I don't have such a complex setup:

iOS version: 14.7.1
Element iOS version : 1.11.4
Selhosted synapse version: 1.95.0

From time to time I get notifications that emit a vibration (phone or apple watch) but I haven't yet managed to identify which ones.

EDIT: I've just tested with 1:1 rooms and it seems to work!
EDIT2: From time to time I have rooms with several people that also work. It's kind of strange.