This is an annoying bug that prevents the user from mounting on the horse, in the realms i was on a horse and i deided to pull up my co-ordinates by pressing [shift]+[f3] (i press shift because on my laptop the f3 button is a dedicated brightness up button)  and whilst pressing this on my horse it brought up my co-ordinates but when i tried to go back on my horse it would let me mount back on, it was like there was no hose there, this bug can easily be fixed by relogging but this is just a temporary fix and its quite annoying having to constantly relog. i apologise for not having any evidence.