When teleported to me after completing a Kamasylvia Main Quest, my horse was teleported underneath the building's floor, and would continue to move under the floor when called while I was still inside the building.
(1) Have a horse/mount accompanying you
(2) Get Quest Item "Kamasylve's Light" from Main Quest "[Kamasylvia] The Healing Light"
(3) Complete follow-up Main Quest "[Kamasylvia] The Heart of Kamasylve"
(4) Speak to Quest Target Brolina Ornette to collect rewards
(5) Observe your horse/mount get spawned under the hall floor (only the "PARKING" arrow is visible, or nothing at all)
[(6) Horse can be whistled out of the floor by moving out of the building and then calling]
Reproducable: ???
Video Link: