I don't see how this would ever be a problem especially if that's a levelling mat. There's always going to be an alarmist option. In this case, if you're bothered by it that much then tear down the entire tank, measure how even the glass is, and request a replacement if it's not to your standard. You can disprove that it has anything to do with the stand by removing not just emptying the tank and measuring how level the stand is.
Either way, if you're bothered by it enough to do something about it then you need to completely tear down the tank and see if you can prove it's the tank that's not even
Either way, if you're bothered by it enough to do something about it then you need to completely tear down the tank and see if you can prove it's the tank that's not even