I’ve been getting a similar issue while mounted and not mounted.
Not consistently, but frequently in SP and MP modes.
It occurs at a transition point between entering the water and swimming. It can happen when entering or exiting the water, but at the transition point between the two.
The horse will suddenly stop when entering the water and fails to continue forward. Forcing me to dismount and return to the shore. (I can happen at any body of water deep enough to swim in)
When not mounted, I’ll become stuck in the transition loop while swimming across the “newb” river in the Exiled Lands.
Enter water, (stop error) transition, swim animation.
It’s not invisible barriers. It occurs at the transition animation between walking, entering the water, begin swimming.
I’ve also noticed a stop error occur when diving for a chest of gold.
I can swim down, open the chest, retrieve the goods … but as soon as I attempt to swim up, I’m stuck unless I move laterally. Then I can swim up. During this time I am not crafting anything in my inventory.
I just ran a small test on intuition …
This occurred both while mounted or not mounted when I happened to be crafting silk from gossamer at that time.
So, on a hunch, I paused the crafting and I was able to enter the water without getting stuck in a loop. While crossing, I started the crafting again and sure enough, at the same rate as silk was being produced in my inventory, I was getting hit with that pause, swim animation. Soon as I paused it, I swam normally again.
I was able to enter the water and swim without issue.
But if I was crafting something in my inventory (silk, twine specifically … maybe other items), I would experience that pausing issue anytime I entered the water. If I did not “hit an invisible wall”, I was certainly stuck in a loop animation of entering the water and swimming.
But if I paused the crafting, I could swim normally.
However, when mounted …
If crafting, the horse will stop as it enters the water. Even if I pause the crafting at that point, the horse will not move forward or backward, but can only turn around … I am forced to dismount.
If I’m crafting when I dismount, I’ll be stuck in that start/stop animation. If I pause the crafting, I’ll swim normally when I dismount.
One more thing. Although it’s not consistent, even when doing nothing but riding, after just entering the water (and sometimes when at the point of transition to exit) it will get “stuck” as mentioned. It can happen anywhere on the Exiled Lands Map.
I haven’t trained a horse yet for Siptah, but I strongly suspect it’s the same as Halk describes.