Hello caitlinb and welcome to the ArtRage forums
It looks from your screenshot that you are zoomed all the way in to the painting? And along the edge of the dress there seem to be semi transparent pixels or at least pixels containing white.
Is it noticeable when viewing the whole painting at its intended size?

In general use I am unable to produce the problem you are seeing.
However, I wonder if you have performed any other actions on the painted area while Transparency Lock was active for that layer, but before using the Airbrush?
One of the quirks of AR is that if you “tidy up” as it were the edge of a painted area with say the Eraser, for example, while Transparency Lock is active, rather than remove pixel colour AR changes those areas to the same colour as the Canvas Colour, which more often than not is very light.
Some other actions that might also lead to a similar result are Cut & Paste, after using the Selection tool and softening paint edges with the Knife tool.
Subsequently when you use the Airbrush in Multiply mode any pixels that changed in colour, as above, will give the impression of a slight halo in the same colour as used by the Airbrush.