Issues with speed - Fenix incorrectly detects speed as zero when the actual speed is relatively high (SUP session)

Yesterday I had a SUP session with relatively constant speed (7 - 8.5 kmph with some intervals with higher pace). The session lasts about 60 minutes with no pause. My Fenix 6 "reports" that my speed was every couple of seconds 0 kmph (the length of zero interval was in range 1 - 4 s). The speed profile in app looks like this

Here is the detail view:

According the gps datafield I had good GPS signal (all bars), but it is definitely not area with clear view to all sides. I was using only GPS alone (not with combination with Galileo or Glonass since form my experience Garmin implementation of proccessing mutliple system at once leads to worse results). Anyone having this issues? It is not first time I have such strange behavior.

After more than one year of active use I am still surprised how many issues these watches have..