Anomalies in Power data from spinning bike (+ Garmin Connect question)

The gym I train at got Spinning-bikes with power meters (technogym group cycle connect) and I could add the powermeter as a sensor to my Fenix 5+ (as well as cadence). I've never used Power meters before for training so I was superexcited.
Everything looked good during cycling, except for occasional sensor drop outs (going to test with different placements of the watch). Not a huge deal, since the sensors reconnects right away.

But when saving the activity I noticed some strange average and max power values.

The data recording showed several issues.
1st test - 10 min with F5+ ;

• 3 spikes up to 56 000 watts within the first 10 min (first at 97 sec)
  
• over 5 power drops (to half power)
  

2nd test - 35 min with F5+;

• 1 sec Power spikes (random times in the first 10min and then every 60 seconds)
  
• 2 sec Power drop (to exactly half value) every 65 seconds
  

I’ve attached 2 graphs of 2[SUP]nd[/SUP] test; one showing the spikes and one where spikes have been corrected so one can see the power drops. (Power is the blue line)

I borrowed a friends F5X on the same spinning bike and experienced no power spikes or half-value drops (but a few sensor drop-outs here as well). This makes me think the problem is not with the spinning bike but with the F5+.

I’m on the 5.58 Beta Firmware, so I’m going to try to roll back to 5.10 and see if that corrects the problem.

PS!
This has now given me a very high FTP, w/kg, and possibly too high cycling VO2max.
How to reset these values to null/blank so that these power anomalies do not affect any future training setup or estimates of performance stats? Or they will just be overwritten when a new estimate of FTP and vo2max is done. ciq.forums.garmin.com/.../1428960.jpg ciq.forums.garmin.com/.../1428961.jpg