Hello!
my set up:
FC 2.1
FC firmware 88m
Graupner MX20 with software version 3

Everything is ok exept telemetry on the MX 20 that display (pressing arrow at the right of ESC button) ) completely wrong values i.e. T1temperature -20°, T2 temperature 117°C ,voltage 2,5 and 7,2 v (while it is 13,5) , rpm 2560, altitude -500 m , speed 327 m/s etc.

FC had Jeti/Hott jumper solder
Fc has cable wired from RX to "T" connector of GR-24
Receiver type : "Graupner Hott" is set in Mk tools

The MX20 configuration was copy from previous working configuration (but with earlier FC firmware, uhmm.....)
CH out type :SUMO 12

BUT....

If I go in MX-20 menu "telemetry" ---> "Setting and data view menu" then MK telemetry datas are correct !

I'm quite sure is a setting in the Mx20 but I cannot find it

I believe you will need to go to V4 on MX-20 and your receiver for it to work with .88m. I've done mine and all is displayed as it should. read the very first sticky post from Holger.