My testing place has ModalEnabled as true so the mobile controls are hidden. a GUI opens a DevProduct purchase, and once you hit either Cancel or Buy Now, the mobile controls appear once again.
This is likely because by default on mobile, your mobile controls go away when purchasing something like a DevProduct. Once closing the purchase, the mobile controls appear even if ModalEnabled was true in the first place.
Place where I found the bug: Project Secret Projections - Roblox
Device: iPhone 6 (I’m assuming this bug appears on all iOS devices and maybe even Android, have not tried it)