AirTags have always been "meh" about "finding" anything in motion. And by that I mean the up close "locate this device". I ASSUME it has to do with the fact that its trying to create a multiple point triangulation using only a single device (eg the phone you are on).
Even an airtag moving a little bit, will give you warnings in find my.
Even an airtag moving a little bit, will give you warnings in find my.