when you apply atonement when renew, flash heal, power word: shield it works like it says it extends atonement all the way up to 30ish seconds. but if you apply atonement with power word: radiance i have noticed atonement buff has to drop down to less than 3 seconds before the divine procession will break it back up. this is with me casting radiance then spamming smite on a target dummy. also ive notice that it if i cast it and hit 4 target dummys and my self with atonement that the buff falls off 2 of the 4 dummys but stays on me and the other two dummys while i am spam casting smite.
Divine procession will only increase the duration of a singular atonement.
What you’re actually noticing is smite is increasing atonement duration on targets other than what you’re physically tracking. Radiance is going to be applying atonement to 5 targets, divine procession likely prioritizing atonements on the priest and those with lower duration. So you cast 1 radiance and then smite 1 will increase the duration of atonement on you by 3 seconds, then smites 2, 3, 4 and 5 will increase the atonement duration of dummy 1, 2, 3, and 4 respectively. Finally, smite 6 will apply to you again.