We just had an order come across from Mexico using UPS Canada Standard. I ran some tests and no matter which address in Mexico I use, they are in fact able to select UPS Canada as their shipping option.
If I use an address from any other country however (besides Canada), they are unable to use UPS Canada and only see the correct options.
Anyone know where to start on this? 
Currently on 1.3.5x
I see this as well, using 2.2.3 Professional