I figured we’d see him in flashbacks with a younger Belle or something but in Keck’s other tweet he tells him to have fun filming with Emilie AND Bobby. So maybe it’s not just in flashback? Rumple wasn’t there when Belle was younger. (That would make it a little creepy lol)
After all, in Beauty and the Beast, Lumiere was with the beast in his castle the whole time, not with Belle when she was younger. She doesn’t meet Lumiere til she goes to the castle and meets the beast. Maybe wherever Rumple ends up, he runs into Lumiere?