A couple of people have reported false disconnects with updated firmware airTags in 1.6.8, we ran tests to improve accuracy in 1.6.9 which is now live.
1.6.9 contains some tweaks to the proximity function and the ability to adjust the grace period before triggers are reported as lost.
Please be aware airTags contain little accelarometers, so they are less likely to disconnect if worn on your person or moving occasionally.
AirTags and similar devices have different modes.
The slowest refresh is an inactive (not moving) AirTag which advertises arround every 15 minutes, this is why it's now the default setting. This means if an AirTag appears, it will be detected quite quickly as we know it's in proximity, but if one disappears it will keep checking for 15 minutes to make sure its really out of proximity (and not just being quiet) before reporting it's missing.