Miche Collapse

I was trying to make Hamelman's ("Bread") Mixed Flour Miche.  It was going fine, a nice active starter got the levain going.  After mixing with 10 minutes of Rubaud's method, a rest, and some folds, the dough seemed to be developing.  After 3 more rounds of folds 30 minutes apart, it seemed pretty strong, especially for ~85% whole grains (15% high gluten, 15% rye, 70% whole wheat at ~85% hydration).  I even got a windowpane.  It puffed up nicely in a towel-lined proofing bowl.   

But when I turned the bowl upside down on the peel, the nice poofy proofed dough collapsed to something slightly thicker than a thick crust pizza,  Needless to say it didn't get much if any oven spring.

Overproofed?  The poke test seemed to suggest that was possible.  Or do I just need more folds?  

I built the levain the previous evening and left it at room temp overnight, ~12 hrs.  I used the proofer setting of our oven (~75-80 ℉) for the bulk fermentation (~3 hrs) and the first hr of proofing; the 2nd hr of proofing was at room temp while I preheated the oven and dutch oven). 

Suggestions, good wishes, and useful incantations all welcome.