This has happened with the newest version of Substance Painter 8.1.0 from the Steam version, on a Windows 10 System.
My model has intersecting surfaces. This is intended and needed. In my case, not all faces are always intersecting in the same spot due to animations. As such, I need to clean up the black AO in some of the intersecting areas.
Prior to version 8.1.0 using an AO channel with "replace" could overwrite the baked AO map on faces with intersections to clean up black spots where needed (the method used can be seen in AOSettings.png and AOReplacement.png). I have done this multiple times in the past and it worked without issues then.
With version 8.1.0 the AO channel is still displayed as overwritten (see NoMultiplyAO.png) as intended. However, the AO in overlapping sections nonetheless remains unaffected (see NoMultiply.png).
The issue is due to AO and not any other channel as can be seen when multiplying the AO of the entire section by 0 (see MultiplyAO.png) and overlaying the color channel with white (see Multiply.png).