knold,
We also had a restriction on use of flex power in the case where anti/deice fluids had been applied and OAT is +6C or below. There was no reference to the type of fluid.
I don't see any mention in the manuals of a reason for this. I've never thought about why, it was just one of a list of cases where flex power was prohibited.
But, now that you mention it, I'd be curious to know.