1) The F. really isn't a good binding in general. I wouldn't go below
the 5-Series Binding of Flow.

2) BUT: The "problem" with your highback is none. It's totally normal
that it comes up again. If it wouldn't come up again, it could "bite" in
the snow and you'd fall. At First (Flow AMP5) I had problems with this
issue (upcoming highback), too, but then I found the clou =)
Just try this: to step in the right binding when the highback is in
"up"-position, move the foot in front of the highback, push it with your
heel down (through moving the foot backwards) and slide in the binding.
You have to try this 10 to 15 times but then it's no problem and you can
handle it fast :-)

Greets!