Strange one-way audio problem

I have all the latest versions and modules.
Everything works great on my system except for one extension, an Snom M3 that has the following problem.

All calls work fine and have two-way audio with the M3, until there is a sip event, such as placing the call on hold, or pressing a dtmf key.
At that moment, the call goes to one-way audio.

Placing the call on hold at the M3 side, and taking it off hold immediately restores two-way audio.
This problem happens every time, its not random.

The M3 and all the sip phones are all on a local network, no nat. Canreinvite = no.
I’m really stumped on this, I can’t find anything wrong in the configuration.

Does this happen to internal calls or only external calls? If only external calls, who is the provider?

The devices are right next to each other, on the same switch.