I am having the same issue. I quit trying to save up skullsworns for fusing a drake, and now I have five 5star skullsworns, but it will not let me use all of them for fusing. It will only let me select 3 after accepting "sacrifice". It has also happened trying to use all of my warmaidens to rank up another champion to level 5 to replace a couple of the skullsworns it won't let me select.... it's only happening with multiple of the fusible  champions... it's aggrivating knowing I leveled them up to sacrifice using all that energy and time I put into them and more I can't even rank up one of my 5 star champions with them.... now i have to work on leveling up other champions to sacrifice, what a waste of time. 

Same here tried to 5 star using 2 different fusion type uncommon, ask if to sacrifice but does not select them after click yes

yeah it seems to be a bug. i submitted a ticket to support and hopefully itll be fixed soon.

Same issue with ragemonger

I had the same issue.  I had to remove all artifacts first.  Cost me a bunch of silver though. I hope I get reimbursed.

Same issue with upgrade 5 star hero