This is a very old glitch that's been around since I started playing Wurm online. When traveling over a mine door whether it's locked or not while riding on a horse, cart or wagon the horse,cart or wagon will disappear and the only way to fix it temporarily is to go completely into the mine and back out and your horse,cart or wagon will reappear but if the mine door is locked and there is no other mine entrance to go into nearby you are stuck riding an invisible horse,cart or wagon until you find an open mine or restart the game. If you leave the invisible horse,cart or wagon you will not be able to remount or access. A simple restart will remedy but it's remains an annoying glitch.
Thanks GMs/programmers and good luck :)