I must be missing someting. When I add a linear gradient (or two as in this case), as soon as I exit the masks function, everything disappears. If I open the masking tool again, I can reselect them (usually only 1 at a time??) but when I export the photo nothing shows up. Any masking edit doesn't "stick" and I have searched everywhere without any luck for an answer.
You are conflating the Overlay with the edits. Looking at your screenshot, it does not appear that you have made any slider edits. That black you see is the Overlay(s) for the two Grads. Looks like you have chosen Black on White for the Overlay. The Overlay just shows where the mask is, it is not an adhustment duch as exposure.