So the issue with an unsupported vertical hole is that the printer really struggles trying to bridge from a supported section (the wall) to one that is in mid air (The yet to be printed hole). There is nothing for the bridge to connect to and the wall of the hole to build on.
The trick here is to “build” a surface in CAD that the printer can bridge correctly on either side (or in several orientations depending on the hole size) that the hole wall can then be printed up from.
A good video describing the issue and different solutions.
