Routing ok after ping from companion, heartbeat not ok

hi,
given the setup

ubuntu 20.4 with mavlink-router <-> usb radio <-> ardupilot pixracer <-> rpi companion with mavlink-router

on the ubuntu side there is a python script using pymavlink listening on incomming messages.
gets the heartbeat, … from fc and of qgroundcontrol when it emits

on the companion side there is a python script using pymavlink sending messages:

  • data_transmission_handshake_send()
  • encapsulated_data_send()
  • ping_send()
  • heartbeat_send()

when using heartbeat_send() (followed by data transmission handshake / encapsulated data)
the heartbeat (followed by data transmission handshake , encapsulated data) are not received

when using ping_send() followed by data transmission handshake / encapsulated data
the heartbeat, data transmission handshake , encapsulated data are received

any idea why the heartbeat & the routing does not work?