Ok, so please be aware that I'm not disagreeing with
@tsedge , but "breathers" exist for a number of different reasons and work in different ways accordingly.
Traditional ICE have breathers to allow the combustion gases that get past the piston rings to escape, and to avoid the build up of liquids such as water in the oil in the crankcase. Clearly that's not the issue with an EV gearbox.
Other requirements for breathers are to allow for expansion of the air in the gearbox and motor to avoid pressurising the system and encouraging leaks around seals, particularly into the system as everything cools.
If there's much gas flow there becomes an issue of separating out any oil mist within the air which is generally not simple.
It's not clear what the current issue is. Is the current assembly "sealed for life" as per most modern gearboxes, or does it already have a breather that is just ineffective at separating out the oil mist? Either way creating an effective solution is not simple without the risk of introducing other issues. MG need to explain further (Ok - even at all would be good).