I've now fused Relickeeper twice without the mission completing. The first time I did was before I was on the quest so I figured that you actually had to have the quest first. So I proceeded to fuse him again and once again it wouldn't complete.

I contacted Plarium Support and after 3 days they gave me this very helpful answer:

"Hello! Thank you for contacting us.

We have thoroughly tested this element of the game and have not found any bugs related to it. Please note that to complete this Mission, it is needed to fuse this Champion. Summoning him from Shards is not counted. 

If you have any other questions, feel free to ask.

Best regards, Maksym"

Yeah, thanks bud. I did fuse him, twice. I did not get him from shards.