Are you looking at the traffic from iOS version 12 and above, as ITP 2.x is applied on that version of OS. I think that will e the first thing to check and if you are still using the third party cookie server we recommend to migrate to the cname tracking and also implement the latest version of experience cloyud ID service which has a solution to over come this impact. you can look for the following article for more details which explains the solution. Releases · Adobe-Marketing-Cloud/id-service · GitHub
interesting: how do you checking, if this is based on the same visitors, it seems you are measuring that on an evar in which you are capturing the userID, are you using custom visitorID, I think validating through DWH would be a good Idea, with visitor ID column.