If you jump on farmland, it turns into dirt, as expected. However, sometimes you can still maintain your y position from when it was still farmland. This can lead to the player being stuck in the block. While you do not fall all the way through to the bottom of the block, you do clip into the very top, causing the player to shake up and down, and makes movement very slow until you leave the block.
Please force a crash by holding F3+C for 10 seconds and attach the resulting crash report here.