Perhaps the tag was in a position where it didn't have a good connection to the network? I'm not seeing the same behaviour on my phone when I compare it to my Macbook, but I don't use AirTags. Sometimes the position of a phone can't be updated right away, usually it's either because the battery is dead or reception is bad in the location where the phone is. I also suspect that a phone that is being used (for an actual phone call) does not update its position as regularly as during other times, but I'm not sure about that.