This may have something to do with it:
https://forums.flightsimulator.com/t/incoming-soaring-feedback-on-the-su12-weather-update/576965
and
You may be experiencing the notorious heat-only thermals in MSFS. General observations are that they are too powerful, they extend too far up, and there are far too many of them. While I do not agree with the opinion that they should be completely removed to appease a few glider pilots, something does need to be done to temper them. You may also notice that if you turn the wind off, the heat-only thermals diminish significantly, which actually makes no sense at all.
Trees are coded to destroy lift, so if you fly over suburban areas with buildings and trees, that incessant bumping is you basically flying through tunnels of lift and sink that are somehow preserved thousands of feet up, even through a wind layer.