Magnolia brooklynensis ‚Yellow Bird‘: Dead branches

In June 2019, the 25-year-old plant first suffered failures on a few branches: the leaves of individual branches died completely within a few days and dried up on the tree. The cross-section of the wood shows no discolorations that could indicate a fungal infection. The rest of the tree is completely green and healthy. The plant is well supplied with water (irrigation at night). Who knows the phenomenon and knows advice?