Why does Jitsi try to use other ports than configured in TURN?

We have a setup where a TURN server with a very limited port-range (50 ports) is behind a firewall in a DMZ.

We do not use a video-bridge and P2P is not possible.

But jitsi still tries to open channels above the limited port-range (which we configured in the turnserver.conf). That results in failed calls.

What is the reason for this and what can we do against this issue?