The differences in line size are too small and the safety margin in the US Sailing recommendation of 72 diameters for bury length is too large to explain the failure.
The failure was due to slippage with minimal load. Stitches, and not a lot of them, would have stopped that happening. Space them 1/2" and do them in both planes with strong thread
A locked Brummel is not necessary; but using on means stitching does not need to be inspected as part of a check on the integrity of the splice.
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