Copter 3.6 Chibios and TBS Crossfire Issue?

Some times I have an issue with Crossfire RX connected with PPM, 12 channels. Now and then it happens, that Copter does not recognize the PPM signal of the Crossfire receiver after powering up.

This happens with a Pixhawk 1 (2.4.6, Holybro Pix32) with Copter 3.6.8 and a TBS Diversity Receiver, but also with a Matek 405-STD running Copter 3.7 master and a MicroRX V2.

The Pixhawk did run with Copter 3.3, 3.4 and Copter 3.5.5 before without any issues or anomalies.

Mavlink telemetry is not affected, Copter gives an “prearm throttle below failsafe” message.

Anyone else with this issue?