I'm having the same issue. I have yet to see any solutions or causes in the forums or anywhere on google. I wish I knew when it started happening, I just know I can't take any damage whatsoever from falling. Armor or not. I recently went to the Nether, built solar panels, turned on fly mode briefly to go look at my solar panels on a hill, but I deactivated it. I can't remember if it started happening before I used fly mode or not, but any light shed on this topic would be great.