There was something very bad about the way xojo draw on windows. During 2018 there was LOTS of changes, this is one of them. It is very common that xojo breaks some things beetwen releases, so, if you are going to move to another release, it is better to look at ALL of the release notes to see the changes.
In 2017r3 labes where custom controls, now they are native ones in windows, also they changed the way transparency works
At least in 2019r1, it is working, the pseudotransparency uses the background of the parent object, maybe you need to recreate the controls again, and be shure that the list is indeed the parent of the labels.
In here, the top list is NOT the parent of the labels. But in the bottom, the list IS the parent of the labels.
Note that this is called pseudotransparency because the label just change the background color to white(same as the parent), but is not really transparent.
My specific case is with regard to placing a Label onto a Listbox with a custom back color drawn in the CellBackgroundPaint event. In that case, the Label is NOT transparent, because it’s not looking at the resulting color assigned to the cells.