Mash thickness should not affect fermentability at all. This is another one of those old wives' tales that just isn't true.
Not sure about this. A brewer friend of mine also does full-volume (or nearly so) BIAB mashes, and his beers are also consistently overly attenuated, and they come across as dry and thin. He is doing everything right, and so I can't help but think that full-volume BIAB mashes are causing this. If infection can be ruled out, then very high attenuation across different yeast strains means that you are consistently making worts with a LOT of fermentable sugars in them. So then we ask: what is happening in your mash that creates so much fermentable sugar?
The idea behind mash thickness affecting fermentability is feedback inhibition, where the product of an enzyme's reactions inhibit the enzyme's function. Feedback inhibition is built in to the biochemistry of all living things, including plant embryos. It's a real thing in the mash. The question is whether it matters, i.e. makes a perceivable difference, amongst variable mash thickness levels. Within a "typical" range of mash thicknesses, probably not, and I think we can write it off as an old wives tale in that context.
But as the magnitude of a variable becomes more extreme, it is more likely to be noticed. With BIAB, mash thickness--or really, thinness--is taken to an extreme. In a thinner mash, there will be less feedback inhibition happening, perhaps enough to be noticeable.
At the mash temps you are using, you are favoring the activity of alpha-amylase. This enzyme cleaves the starches indiscriminately until all that's left of the starch chains are limit dextrins. Although the conventional wisdom is that higher mash temps will result in less fermentable wort, it is possible that high alpha-amylase activity for 60 min combined with a thin wort (i.e. little feedback inhibition) is resulting in most of the starches and oligosaccharides being cleaved down to glucose and maltose. Hence, a very fermentable wort.
I doubt this issue is being caused by hop creep or other strains hiding out in your equipment. I suggest trying a more traditional liquor-to-grist ratio in your mash and see if that makes a difference.