I’ve seen it that too. Never before experienced that. It almost feels like it’s positioning itself against wrong viewport coordinates or something (eventho the translate3d coordinates are correct in the inspector, they don’t match the actual position) and then we re-applying the property, it has rectified itself.
Safari has a whole bunch of mentions of background-attached:fixed not working properly, could be related to that somehow. None of the speculation offers a fix tho 
We could change the dropdown to position:absolute, without a translate, that works. The way it’s done now is a bit weird to begin with imo.