I'm frankly having a hard time understanding how the bottom entry opening could shift (or roll, which is what you are describing). I have three BIAS bugnets (two Buginators and a NanoBuginator) and have never experienced this issue. My kids have used them and they can break the design of anything, but haven't found fault or been able to destroy the BIAS Buginator.
The only thing I can think of is that you have no hammock ridgeline. Is that the case? If you have a hammock ridgeline, are you cinching the ridgeline openings of the bugnet with shock cord or something? That should minimize bugnet roll.
I suspect your ridgeline openings are rather large and uncinched, though I could be wrong. I also suspect your dimensions are not tapered from ridgeline to shock-cord bottom opening, but somewhat straight. Again, this is just speculation.
Can you give us a visual on that bugnet, a pic or vid? A picture is worth a thousand words.
By the way, I pay not attention to the size of the bottom entry opening - it could be the size of a dinner plate or the size of Roseanne Barr's backside. I don't even try to get it snug. I don't think it's in the DNA of a mosquito to search for food that way. I am mosquito-phobic and have never had a mosquito come in through the bottom entry no matter how loose it was cinched.