Symmetry Glitch when creating Sub-assembly

Mirror a fuel tank on the side of a fuel tank to make a cross-like shape.

Attach a second fuel tank onto the mirrored piece.

Detach the second tank and directly drag it into the "Create Sub-assembly" box.

The side you used to create the assembly will still have the two segments, but the opposite side will only have one. Symmetry still exists but the one part that was dragged is missing.

You can then continue to add segments, and they appear on both sides, but the missing segment will not reappear.