ComboBox Dropdown not opening in "Client" mode with "Run Under One Process"

Recently when I try and open a combobox (both custom and default) in a client session with run_under_one_process enabled, the combobox never reaches the point where it opens.

I’ve traced the problem back to the following:

SMenuAnchor::SetIsOpen →
FSlateApplication::Get().GeneratePathToWidgetUnchecked(AsShared(), MyWidgetPath); →
FSlateWindowHelper::FindPathToWidget

Then FindPathToWidget never successfully returns a path, causing SetIsOpen() to never complete.

I’m not sure why FindPathToWidget is failing. Specifically in RunUnderOneProcess but if anyone has any ideas please let me know!