Strange figures, but without doing a logarithmic calculation I suspect your untweaked sync-time (target) margin is 9dB, and you are tweaking to 66% of that. Remember, 50% of 9dB is 6dB.
As long as you don't do several re-sync's in a short time I wouldn't expect the MSAN/DSLAM to react by raising the target margin.
That you should then have had a disconnection sounds more like coincidence to me. Some other event causing it.
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