Garmin 955 solar sometimes showed bad cadence pattern and zero cadence on running. Please suggest

I have exactly the same issue, didn’t know if I needed a software update as I know I run between 175-185 but it’s showing over 200, then down near 100

Look at this another problem with cadence. 

Interval training.12.27 firmware. 
Cadence stops registering if you suddenly start moving fast. But now the pulse also stops being registered!

Does this mean that pulse measurements are not taken from the sensors, but are only calculated depending on the cadence? I can assume that the Garmin uses such a small trick when the cadence is very high and it is difficult for him to measure the heart rate. But this leads to doubt whether the heart rate readings are reliable during the entire workout on the watch.

For the test, at the end of the workout, I accelerated not so sharply - and the cadence and pulse were shown accordingly.