Sounds like these can just be 1 of those mystery deaths that we'll never discover. Sorry you've been having bad luck. Maybe the 2 were already carrying something that you just couldn't see
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+1 If you did everything correctly as it seems you did, then it seems that their deaths were not preventable. Meaning whatever killed them, came from a different point of origin...since you are positive your parameters were perfect. I would investigate the LFS or source of these bichers.