Several with the pumps installed wrongly - they need to be at a sufficient angle to clear any bubbles within the pump, but not at too much of an angle - or they are likely to generate more gas bubbles due to cavitation on the suction side. 35degrees upwards sems to be the preferred angle but vertical also seems acceptable (most certainly better than less than 15 degrees!). Bubbles should only be from more volatile dissolved shorter-chain alkanes (mostly), not from air leakage into the system.
It's not so much air bubbles, it's the fact the pump will likely just not work.
It's a basic solenoid plunger pump with weak return springs, so mount it upside down, and it just won't pump. It needs gravity to make it work correctly.