I'm assuming that the devs forgot to send a packet for actually getting off the horse but show it to the player or something like that (I had it happen in singleplayer too, and singleplayer uses an integrated server so)
@Galaxy_2Alex I tried to use the search function and after fighting with it for about 30 minutes I couldn't find anything. It's clunky and dysfunctional. Sorry for the duplicate though.
@Aiden I beg you to not be so quick in your ruling. What kind of texture pack changes the model for the dirt block, making it lower, and having the farmland texture, let the user be able to put vegetation on it, and then have it go dry if there isn't a water source nearby? I'd like to hear of it, please.
On a more serious note, you can clearly see that the block is lower than the normal dirt in the attached picture.