Pokemon-go – Why did the distance requirement for the buddy change

Anytime the "loading" symbol appears in the corner, your distance is usually not being counted. If you wait for the game to finish loading before you walk, your distance will almost always count. The update is dependent on whether it is able to get a good GPS signal but even if you have excellent signal, if the game is loading it will not update until it has finished loading. At least, according to a number of experiments done by a few people that seem to show that distance is not counted while the game is loading. This would explain the jumps you experience as well, I would think. Hope this helps.

Niantic is rather clear in the Google Play Store:

• Compatibility with tablet devices is not guaranteed.

For whatever reason, they decided that your tablet does not "work" with the game for whatever reason, and have since blacklisted it from being used on those tablets (even though they meet/surpass the actual requirements for running the game).

I'd contact Niantic Support and ask them why your tablet is explicitly blacklisted. It may very well be a mistake on the part of the last published version of the app.