Side by side QComboBox dropdown display at wrong coordinates

The next to last (i'm only using two) side  by side QComboBox control displays the drop down at the wrong coordinates. I've tested this by switching places and the next to last control consistently displays at the wrong coordinates. Initially, my configuration used a QToolbar for each control in which case both controls showed incorrect dropdown displays. However, after combining the two controls on the same toolbar the last combo behaved as expected but the combo to the left consistently behaved as described.

I'm attempting to upgrade from  Qt5.11.3 (MSVC 2015). The behaviour described does not exist on this platform.

I've attached as shot where you can see the behaviour described...

... and another showing the declaration and toolbar assignment code...

Cheers,