The flight does appear to be a 747.
Where on MMB is it saying 777? I have noticed a glitch on the system if the equipment changes. On an upcoming flight I have, on the seat map screen it clearly shows a 747 layout, but says 777. The flight is on a 747 and under 'more flight information' it has the correct type, 747.
Perhaps this is what you are looking at and seeing 777? Is the seat map in MMB showing a 747 layout?