Pocket Edition GUI.
Very probably related to MCPE-20351 raised on furnaces.
Just previous to the test screenshots below I noticed the inventory fence slot contents replace all four of the maps I had in hotbar slots 6-9. In inventory (I got off the horse to do this) I reorganised the four maps into inventory slot 6-9 and resynced hotbar slots 6-9 to inventory slots 6-9.
Testing from the above point onwards I re-mounted the horse and simply entered and exited the horse GUI a few times without moving anything in inventory.
Shot 1, note hotbar slot 4,5 with same instance of axe from inventory.
Shot 2, In Horse GUI, note inv slots 4,5, & 15.
Shot 3, note hotbar slot 4,5 now with same instance of leads from inv replacing axe
Shot 4, In Horse GUI, note suffling of inv slots 4,5 & 15.
Shot 5, hotbar slots 4,5 now with same instance of raw chicken replaces leads in hotbar.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-14828.
It has been linked to this report. If you have additional information, please add it to that report.
If you have not, please search before reporting, as it's likely that one exists already.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Feedback – 📖 Game Wiki