Undo in Designer causes parts to appear in strange places

The struts showing up around the outside of the fan enclosure seem to be phantoms of the half of the set of struts that make up the grill.

How did we get here?

1. I created half of the struts for the grill (the left hand half as we're looking), manually positioning and sizing each one.
2. I attached those 7 struts in a chain tip to base from the outside to the insite.
3. I attached the inner most strut tip to a fuel tank (since removed) that I attached to the command chip.
4. I used the mirror tool to mirror the 7 struts from the left half to the right half.
5. I attached the inner mosted of the mirrored struts to the fuel tank and made the whole thing into a subasembly.
6. I added another copy of the sub assembly, postioned it, rotated it 90 degrees
7. I then broke the connections to the fuel tanks and deleted the fuel tanks.

... I did some other stuff, not relevant

1. I tried to add a fuel tank which inadvertently connected to my propeller blades and got radially mirrored.
2. I hit undo.

Boom, instead of just removing the fuel tank I just added, I wound up with this.

I think these struts are literally phantoms, with the same part id as the actual struts in the proper place. I deduce this because despite having no other connections, when I delete the phantom struts it deletes the real ones to. I will try to confirm.