Short update while doing more testing, I guess I figured out the cause.
We have two core nodes using the dialog module and:
modparam("dialog", "enable_dmq", 1)
If the call is handled on core1 and the session timer is increased, indeed there is no hint of the dialogue expiring.
But now I had a look on core2 and this is where the call is expiring.
So I guess the refreshing of the session timer via AVP is not DMQ synced to the other node? Should I replace the AVP by a DLG var?