Hi Clemens,
I would recommend reaching out to Campaign Support and providing them with more details around the reporting and what you are experiencing. We typically don't need to perform ad-hoc updates to the classifications, so something else is likely going on.
One thing to note is that Apple's newest iOS 15 allows devices not to be tracked, so if you have a high concentration of iOS users who have adopted iOS 15, this may be the cause of the increase.
https://experienceleague.adobe.com/docs/deliverability-learn/deliverability-best-practice-guide/addi...
Regards,
Craig