if this were the case, wouldn't all BIAB brewers have this problem?
In principle, yes, if they mashed at 158 for an hour with a very thin mash. But I don't know many brewers who regularly mash at 158, and so a critical mass of observations may not exist.
I'm not convinced that what I'm suggesting is the case here. It's just one possibility out of many. But this poster's worts are apparently highly fermentable, which, assuming other possibilities are ruled out*, means that those worts have a sugar:dextrin ratio strongly tilted toward fermentable sugars. This points to a very high degree of enzymatic activity.
*and this admittedly is a big assumption and is not necessarily the case