Idk why people keep thinking Roman forgot about Seths betrayal. He didn't, he just approached it from a different angle.
Dean went after Seth because he was like 'wtf I can't believe you betrayed us'
Roman went after HHH and Randy because he was like 'wtf this is all your fault you corrupted my mans'
Roman was always the most level head and mediator in the Shield so the paths they took post-breakup made sense.
This was like 3-4 months AFTER they broke up:
So it's clear Roman never wanted to go after Seth in the first place; he wasn't the root of the problem...the Authority was.
The story was there y'all just turned a blind eye to it