Makes sense given how map5 is programmed. If you're seeing timing drops map5 will continue to lower it's boost target until they subside or it hits the floor at 11psi. At elevation the factory boost target is around 10psi, and you're seeing drops there as well, so map5 is just going to drop down to the min level. In situations like yours (high elevation / poor gas) richening up the air/fuel ratio can help. It's under user settings.
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