At the suggestion of Robert Lupton we should confirm that this is the expected behavior. The code in question is attempting to fit a child's flux to a multiple of the PSF. It appears that this can fail if the model doesn't match the PSF. The value of `rcond` determines what to do with singular values, which appears to have changed and has triggered this warning, which is itself in a try except block.
I would recommend not spending too much (any) time on this. This code is not used at all in the new deblender and the time it would take to sufficiently test and understand this behavior does not seem like a good use of time.