Buffy the Vampire Slayer moved to UPN from season 6, while Angel stayed on The WB. Crossovers were complicated because of contracts. Some happened, but not with the biggest stars or for long. AA’s big Buffy/Angel reunion wasn’t possible during seasons 6 and 3, so it happened off-screen.
We don’t know what happened when Buffy and Angel reunited after her death. One theory makes it even more powerful: Buffy’s memories of their day together in “I Will Remember You” were restored. Since Buffy went to Heaven, she could remember her happiest moments and see what happened to her and those she cared about. This would make her memories return, so when she was resurrected, she returned with them.
There’s no confirmation, but the theory fits the missing scene.
The Buffy comics didn’t explain what happened when Buffy and Angel reunited.
After Buffy and Angel reunite, the Scooby Gang wonders what happened between them. Xander imagines Angel in danger and needing Buffy’s help. Dawn imagines a romantic meeting. Anya imagines Buffy telling Angel a secret at a Denny’s. But a fire demon interrupts. It came from a spell by Willow. It gets into their minds. Buffy stops it in the real world.
This comic shows that the Buffy the Vampire Slayer team didn’t want audiences to know what happened between Buffy and Angel after her resurrection. It would’ve been tremendous, and it would’ve been hard to live up to expectations.