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Are you sure the setpoint is at exactly 90%? You can sometimes fat finger the little slider and be off by a few percent and not have it be immediately apparent.
You should probably monitor it at the very least. I think there may be some additional diagnostics they can perform. It seems reasonable to push for that if Tesla tries to close the issue with a generic statement that "degradation is normal". I will be honest here, though, and say that most of these types of cases we have seen here on the boards were the result of a new user misreading or misunderstanding what they were looking at (ideal versus rated etc). But there are also cases where there was a real problem that was eventually ferreted out. I only bring that last nuance up in the hopes that, as you investigate further, it ends well!