Ah, that will likely be a safety feature then, as it's important that the seat sits flush against the backrest, it is part of the rebound protection. You can't normally move the isofix for more leg room unfortunately. That is a very rare feature which isn't offered in most car seats, and usually when it is, there is a different anti-rebound measure instead to make up for it.
That said, it's not a problem for children to have their legs touching the backrest of the seat. As long as she is not uncomfortable, and most children aren't, it's not a safety problem for her to sit that way.
If you want her to have more leg room then you need one of the Swedish type seats like the Britax Safe-Way M or Axkid Minikid 2.
I'm not sure why it locks into place in the forward facing position but not the rear facing position. Usually, the reason that there are multiple possible lengths for the isofix arms are because different cars have the isofix attachment points at different lengths so you need to be able to reach them in cars where they are deeper, but in normal use the child seat should be pushed tightly into the vehicle seat. It might be that the top tether is doing the work there with the forward facing mode so it doesn't matter as much, but I'm just speculating. I can't find the manual of this seat for download anywhere.
Does the manual say anything useful about the isofix arms or about leg room?