I encountered this bug on accident. When casually exploring the nether in a new world that I had created in of the early snapshots (I think 20w09a) but hadn't really used since then I found a group of Striders in a lava-lake. I accidentally punched one of them and they somehow gained enough upward acceleration to leave the lava and float up to the ceiling of this cave. However, they (all of them showed the same behaviour when punching them) never came down again. They were completely unaffected by gravity. They weren't static, as they could be pushed around horizontally, but they were stuck at this exact height. Attached is video evidence (I immediately started recording...). https://drive.google.com/file/d/1J2RrfgCWRlnWiopq49P6YY-GRLQcYcXR/view?usp=sharing (The video was too large to share via Jira).