Yes that is the way it works. It is exactly the way it was in PL4 except for the addition of Control Lines and the Chroma/Luma sliders.
Lowering the opacity of a mask just makes the mask more transparent, and as a result you will see what is beneath it. You can’t unpaint a portion of one mask using another mask with lowered opacity. Erasing parts of a mask differs depending on the type of local adjustment. To delete parts of a control line or control point mask you need to use negative control lines and negative control points. To delete parts of the others LA masks you can use the LA eraser tool.Not sure how the CTRL key would be used in this situation. Did you mean the Alt key? The Alt key is used to create negative control line and control points in the Windows version.
UPDATE: This is definitely a bug as adding to a mask often results in the added area not showing the same adjustment as the original area. I find I have to delete the mask and start again because restarting PL5 does not result in correct rendering of the local adjustment.
I would like to clarify one way I can guarantee of showing this problem.
If I paint a mask and then make the selection blurred, there is no problem when adding to the mask.
What does cause a problem is using the eraser and then trying to repaint an area or add to the selection after the eraser has been used; in which case, the repainted mask doesn’t adopt the blur that applies to the rest of the mask.
This has already been noted and a fix is on its way. In the meantime, if this happens, a simple fix is to adjust the Vibrance manually to something then return it to 0 with either the mouse or the up/down arrow keys.