The shelf life of Lithium coin batteries is about 10 years, so even if these have been sitting around for 1-2 years, you realistically shouldn't see any noticeable reduction of battery life. Energizer claims that their CR2032 loses about 1% per year in storage, resulting in 90% capacity at the end of the 10 year shelf life. I don't know what brand is inside AirTags but I assume they're pretty similar.Sooo are the batteries inside not at 100% then?
Lot of info in this document. Page 7 is about shelf life: