These cracks occur when an instrument is assembled or when the tenon is swollen, due to high humidity. Socket walls are very thin. If you have problems assembling the instrument, you might be using too much force and therefore splitting the socket. Only the metal ring holds the wood together. If this happens a metal receiver is a good solution.
Sockets with metal receivers also show occasional cracks. The wood wall here is extremely thin. The reason is the same, but these are just optical and it is safe to simply re-glue the crack. They have no acoustical effect.
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