Sorry of this has already been posted, but I couldn't find it using just the mobile site. Anyway, using a controller during gameplay seems to decrease the number of hotbar slots available if you start using touch controls again (at least on high definition ). Quitting to the menu and reloading that world/server will bring back the missing spot until the controller is used again

The first picture is me getting on my friend's LAN world and not touching a button on my controller, second is the hotbar while using the controller, third is the hotbar after switching back to touch controls. It is missing a hotbar slot

To reproduce, load a world, and take note of the number of hotbar slots, connect and use a controller (which takes off the ... slot), and then go back to touch controls (the ... slot overrides the previous ninth slot)