Bike - Speed sensor leaves gaps in speed chart

I have a few of those but to me they are logical because they coincide when I brake at zero km/h. Maybe on the one I'm at 20Km/h and I slam on the brakes, then on the next record in the log it's at zero and then it's like that for a couple of seconds while I was braking and if I get out fast the next one will be something > 15 km/h as one accelerates fast. I understand that this is personal and to get your doubts out you should do a known route and know where you brake, speeds, etc. You should check it based on your driving and trail, since what the sensor measures is the rotation of the wheel, if for something this decelerates suddenly or stops turning it could take that value I understand.
As for the AUTO setting I don't think it will give you problems, it works very well, although I'm a bit of a maniac and I add the value measured by me of my wheel since each tire differs a few millimeters and it is an easy task to perform.
I use them on the front wheels of my bikes but I have always used the original Garmin sensor, I don't know how the others work.

in some cases the electromagnetic field of the cables under the roads can fool these new type of speed sensors. maybe you are riding on these type of roads. does it happen randomly or always on the same place? (in this tiny pic the 1st half has some issue, later it is ok). and do not use auto option, set it to manual, sometimes the bad gps reception can fool also this wheel circumference value.

By my experience so far, it happens when I go through bumps (jump?), holes, irregularities on the streets. (I'm using a MTB).

Like if I were in an off-road trail, are speed sensors suitable for those? I understand that the wheel is expected to move irregularly on bumps. And the sensor might take a few moments to identify the round movement pattern again. But for those moments I would expect the algorithm to identify and correct these artifacts. By the way, when I checked the same activities in Strava (auto sync from Garmin), it shows smooth graph/chart. 

About the "auto" option, I like using it because even if I do a perfect manual measurement of the wheel, it might change along with the tire pressure, temperature, environment, ambient pressure, altitude, etc. So my guess is it's better to have it always adjusting itself.

With another brand sensor it seens better. Maybe is a fault of these cheap sensor using the same sensor for cadence and speed.