My first build was on a Schwinn Alloy Seven, which was a 6061 T6 aluminum version of the classic 1955 cantilever Schwinn design.
My frame cracked in the very same place, and I found it the same way OP did. Bike felt loose all of a sudden, so I stopped and had a look.
Sadness. I ended up moving everything, wheels included, to a new frame. A steel frame.
It is my belief that aluminum frames are generally unsuitable for MaBing in the long term, especially with a chinagirl. Aluminum frames fatigue much quicker than steel.
One thing's for sure: I never liked any of Grubee's bike frames. They remind me of older Hyundais and Kias: they look okay, but upon further study are rather shoddy.