Your username: YouFoundGolem In which minigame/server does the bug occur: Peaceful Survival Detailed description of bug: After a few seconds of riding a horse, it will sink down in the ground and one you dismount from the horse, it will go back to the spot you were just in... Spoiler: Video
Try using a different version of your minecraft. If this doesn't work, it may be the chunk of the world you're in, I didn't have this issue. If neither of these options work, I would suggest contacting staff.
A tip to everyone. If you are using a higher version than 1.8.9 the server might have some issues so please do downgrade to 1.8.9 for better gameplay and no issues.
My brother and I were both having that exact issue and we've been playing in 1.12 recently. I'll try 1.8 and see if it works better. Edit: It helped a lot. Thank you.