https://www.youtube.com/watch?v=jZzlkWVupeY
The button to register a horse to the stable is interupting the UI to transport a horse when it's the horse at the bottom of the page. Pressing the exact amount of clicks you would always need to in order to transport a horse by muscle memory will result in you using a reset mount growth pearl item if you have one. This needs an immediate fix as the ToS/operational policy does not cover this if you lose your reset pearl item or horse progress.
You can scroll down to reset mount, click down once more(where you'd expect to move to TRANSPORT) but instead its interupted by the register mount UI and still counts as being on "reset"