Please try this:
1. Remove SWM - ChooseStarter.rb (moving it to the desktop will be fine)
2. Install SWM - ChooseStarter - debug mode.rb
(It's essentially the same mod, except that this one will verbosely say what it is doing when checking for the starter room)
(It will also confirm that it has been loaded on game launch)
3. Start a new game
4. Go to the starter room
5. Choose any pokemon
6. Open the party menu
What messages did the mod display?
It's an intended change.
The mod will now also lower EVs to ensure that the 510 limit is always being respected when required (it will try to respect obvious attempts at EV training though, when deciding what to lower).