Curtis Gibeault

I looked at your world. I am surprised to discover that corner blocks allow you to exceed the normal maximum of 64 blocks horizontally (x or z) for structure blocks (if you try manually setting a size too big in the structure block itself, it changes it to 64 on the x or z). It seems that corner blocks too have a maximum, but it's instead at 79 blocks horizontally. I tried placing a corner block at 67 -19 71 and it detected it, placing the z size at 79. Any further away and it wouldn't detect.
Also, you only need the corner block that is in the opposite corner (in all dimensions) from the structure block (you don't need one at every corner). For example, looking at TowerMansionV4, its structure block is at 109 -62 0. You'd only need the one corner block at 145 -29 44 to select the entire structure.

I stopped using the keybind while it was bugged and was waiting to see it fixed the patch notes, but I just noticed recently that it is no longer resetting to the default. I'm not sure which version it was, but this issue appears to have been fixed.