I have a rogue who just hit 3rd level and went Shadowcaster, and one of the spells he took was Mage Armor. He cast it on himself, and then cast it on my Wizard, to save one of my Wizard's spell slots, and the Mage Armor on my rogue ended. Mage Armor isn't a Concentration spell, it's an 8 hour duration spell, so I should be able to cast it on multiple people if I have the spell slots available.
It happens with spell slots too. I just tested it as well. tbh I always assumed Mage armor was always a "self" spell. BUT... with the wording of the actual spell from the PH "The spell ends it if the target dons armor or if you dismiss the spell as an action." could mean Mage Armor is an actual armor piece so you are magically transferring it onto another person.
Could be a bug or could be how they interpret it
It's definitely not an actual armor piece, RAW in 5E. It's just a magical field, and in 5E there's no prohibition on casting the spell on multiple people using multiple spell slots or items.
I think this is just a bug, which I expect will be easy for them to fix when they eventually take a look at it.
There is one other limitation of having one proxy spell active. if mage armour is typed as a proxy spell you wouldnt be able to have 2 of them or any other proxy spell. so not flaming sphere and mage armour.