Hi Mauricio,
It truly seems to be an error that your _ah/stop handler is not being triggered, and that there is no info in the logs explaining why your instances spin down. As you say, you've already got a case open with support, so I suggest that you continue to follow-up with them there, and include any relevant details they would want to know in your responses.
Unfortunately, this forum isn't meant for 1-on-1 support or specific technical issues, but is rather meant for general discussion of the platform, so as said above, you should continue to look into this issue in the correct way, which is with 1-on-1 support through a support package.
Best wishes,
Nick