That is really annoying 
You can see where the problem is, that disabled layer-group layer-mask, but I can not at the moment reproduce that in a simplified form.
Ok working in 32 bit fp linear but do you need an Alpha channel in the png ? If not, export as either 8 or 16 bit RGB That drop down menu in the png export dialogue. (8 bit is probably adequate and a smaller file size)
Another way is introduce a temporary 'new-from-visible' layer above the layer-group and export that.
You can see where the problem is, that disabled layer-group layer-mask, but I can not at the moment reproduce that in a simplified form.Ok working in 32 bit fp linear but do you need an Alpha channel in the png ? If not, export as either 8 or 16 bit RGB That drop down menu in the png export dialogue. (8 bit is probably adequate and a smaller file size)
Another way is introduce a temporary 'new-from-visible' layer above the layer-group and export that.