Those are cracks formed as the result of localized stress...I've seen hundreds of these. Note the finish is lifting primarily off of the cap side of the fissure and less so on the neck side. This indicates not only a tiny relative movement between these parts but also the direction of movement. The finish stays on the neck because of the relative direction. On the neck side of the fissure, the finish is being pulled indirectly back toward the neck and on the cap it is being pulled indirectly away from the cap.
Local stress....
The cracks may indicate too much movement of the neck right at the heel....but not necessarily.Was the cap glued to the neck prior to joining it to the body? If so....the glue line of the cap to the neck and the glue line of the cap to the body are two seperate gluing operations, which allows for the possibility of having made some kind of gluing error when the cap was glued to the neck.
Here's a theory. I'm guessing you glued the cap to the neck prior to setting the neck into the body. The glue joint of the neck set has integrity. The glue joint of the cap is compromised. When the strings have tension the neck bends forward and applies the kind of stress indicated. PROOF of this theory would be if someone noticed that the cracks began at the tip of the cap and then propagated toward the body over time. I'd ask the customer if he noticed this. That would be a 100% indication that the neck has localized stress at the heel....not necessarily a disaster....but not happy either. If this is the case then it begs the point that the cap area has a lot of stress there and in order for this geometry to work the bond of the cap to the neck must be very solid.
An interesting observation might be to let it go for a while and see if the neck joint holds up over time by itself as the stress becomes more concentrated on the neck set as the bond between the cap and neck continues to fail. If the neck set holds up then the answer for this might be to make the heel a little deeper next time and make that cap much thinner. This would relieve lots of stress between the neck and cap for the given movement in that area.
