iOS and Android modules 'uninstall' themselves every couple of days (MacOS)

About a week and a half ago I opened my current project in Unity and got an error message telling me that the project was configured for iOS player and that wasn't an available option. I was very confused because I'd literally just built to iOS a couple of hours earlier. I 'Switched Player' to the PC option and then went to Build Settings and sure enough iOS was reporting No iOS module loaded with button Open Download Page. Android had the same thing (despite having been installed previously). I needed to get a build up so I ran through the (multiple GB) download and install process and it was working again. Until a couple of days later when the EXACT same thing happened. Since then I've installed the latest Unity Editor (2018.3.8f1) and this morning... yep you guessed it they've gone AWOL again.

Running MacOS Mojave 10.14.2 with 70Gb free space on my drive (ie there shouldn't be any automated 'clean up' happening to the best of my knowledge).

Hoping someone knows what the hell is going on and can provide me a way to stop this happening because my project is due in two weeks and we're on a bloody tight timeline as it is...

 

Interesting discovery! Switching branches in git 'restores' the modules somehow. Has to be something to do with the project settings because the installation is in the Apps folder and the project is in the Documents folder...