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We don't know if there is any shim inductor, or,  how high the pri side leakage is on the Tx  -  you can see in the waveform  ( just  -  LHS )  that the transition at the end of the power pulse  ( some times called the leading leg transition )  IS  ZVS,   but after the circulating current period there is insufficient energy for ZVS on the  passive to active transition  ( sometimes called the lagging leg transition )     -    this can easily happen at mid load  for a given design  -  and appears to be happening here.