My guess is it's related to the drop of WP8.0 support from 3.6.
It removed the 'm' identifier from ContentManager, thous also removing support for XNA's XNBs.
@DDReaper, Maybe we should add back the identifier like we did for legacy platforms in the past.
I see that MonoGame still support XNBs from XNA/Desktop and XNA/X360 projects ('w' & 'x' identifiers).
Along those lines, 'm' should be added back, not as a legacy MonoGame platform, but on the very top of the list along with the other original XNA plaforms.