Sometimes some Mobs in the world, such as horses, dogs and sheeps are like they are frozen, they don´t move, sometimes (not everytime) you can´t attack them, you can´t puch them in any directions. If your sitting on a "frozen" horse, it will move a little bit on his location, if you try to controll it, the horse don´t move from the place. Only restarting the world helps temporarily. I don´t know, what could trigger this bug. (I also found the bug on the android version from bedrock, OS is android 9, Minecraft version is 1.16.100.)