When using a structure block it will no longer show any outlines nor any hidden blocks. This becomes apparent when using it for both saving and loading.

I've attached a screenshot which shows the situation right after I set up 2 corner blocks and let my Structure save block successfully detect it. After that happens you can't hide the outline, it will show no matter what. Yet as you can see in the screenshot it no longer does.

For the record: this did work in the previous snapshot.