I'd wipe the wood with plain alcohol and see what happens. If your wood turns red in the same places - its in the wood. If not, the problem is with the shellac.

If it's the shellac (doubtful), try a wax-free shellac (shellac.com - buy the flakes and mix small amounts yourself)

It's more likely that it's the wood. You might try a light stain to even out the color.

Good luck and let us know what happens.

Thanks for your thoughts. Turns out it was stripping residue (I used a water-based stripper). Ugh. I've got to wait til spring to take the doors outside and go over them with mineral spirits to remove it.