Hi Haze, I've just played MAME again and I'm sure you are now correct.
I'm not entirely convinced that my PCB and the PCB on the video I sent yesterday are originals (although they look like it).
So, the 'priority' may be something the copy boards missed out since this behavior is present on at least two different boards.
When those winged aliens go underneath certain areas, you can bomb the background to reveal the winged alien fully. You can then dock with these winged aliens (by bombing them). Therefore, it is likely a gameplay feature that you need to destroy the background (in certain areas) to then be able to dock with the winged alien.
Possibly worth putting a comment in the driver to mention this explicitly - ignore the behavior on copy boards.
We need someone with a known original to absolutely confirm it but I'd say you are correct.
God, I miss those dark, musty arcades ....