Mask edges visible

[Bellus PC]

Very weird anomaly that is becoming increasingly distracting while retouching product photos. Started to notice it a about 3-4 weeks ago. I haven't been  able to google out an answer so far. 
Image is in Adobe RGB mode, mask edge has been erased with a soft continuous brush, changing brush transparency. The mask has rough and noticebale edge, no matter what I do - select and mask, feathering. Using color picker on flattened image shows the value is all channels 0 on the edges so prints are actually OK. I checked and this shows up on old files as well.
Does anyone else has the same problem or a clue what it could be? Resutl ofa buggy update?