1st 20sec strides lap with incorrect avg run cadence

1 more thing to add there..... Normalized Power for that same 1st lap is blank 

I checked the FIT file from 16/07. The average for the first lap is correct. The lap is 20s long, but the first 10 seconds you did not move at all (cadence 0), hence the average of 91 spm is perfectly credible:

hmmm seems like some of the data is not recording (maybe gps is not ready), as i checked in garmin connect web, and pace and elevation are there, hr is at least for 1 strides session, without that break, but cadence, stride length etc are with gap for first few seconds (or like there is 1-3 secs of data, then gap then again data).....

But well.... i'm pretty sure that i was moving each time :)

But well, i think the easiest solution will be to add warm up or other step before that repeats, so it should show correctly all laps of strides (especially that i can just remove that 1st step in the data later in Python or Power BI)

... in fact, after looking at the pace, it looks like you were already running while the watch recorded the zero cadence during the first 10s. In fact, it works in similar way for steps counting - the counter only start to increment the steps after some 10s of continuous cadence. Though the steps counter at least increments the steps from the initial period anyway. It does not seem to be the case for the cadence, with your watch.

I checked couple of my activities (using Instinct 3), and the Cadence is always present since the beginning. Check couple of other activities, where you know you moved since the beginning, and see whether it shows some cadence for the initial few seconds. If not, then report it to the Support (or even better to the bug section of Beta Program forum, if you are using a beta version of the firmware - there is a higher chance it will be seen by the developers).

As a workaround, simply make sure you walk or run at least 10s, before you start the first stride. Adjust your workout accordingly.