I always consider these are in the wood. Finish doesn't just crack without having the wood move under it. But on anything basswood and especially plate joint as said, it's not a question of if but when.
I've see them repaired by Hoshino, you can always tell. Leave it alone.
Between the finish and the wood something must have moved relative to the other to cause a crack. That is for sure. But when I take the neck off there's no visible cracking or separation on the inside of the neck pocket on mine.
The hairline crack in the finish starts from the point of the angle and the wood there internally appears intact.
Mines been like that for over a decade. Hasn't affected tuning stability or playability and I'm never selling it so I don't really care.
Would be interesting to know what's actually going on though. Has anyone ever stripped one down to see?
My other square heel joint of a similar age is a PGM100 which has has the finish removed and its neck joint is immaculate.
But I don't know if it had finish cracks before as it was the previous over who sanded it back. So it's not a real test..