Tfmini and pixracer strange behavior

My tfmini (serial Telemetry1) works only when I first power the pixracer (copter 3.6.6) and after several seconds power the tfmini. If I power both simultaneously I get sonarrange = 0. I don’t get any messages such as Bad Lidar Health as long as the lidar is powered. I have played with the TELEM_DELAY parameter with no success. Anyone know what could cause this problem ?