(For reference, Carver is the PC that disappeared and can't be added. The protagonist NPC is Solace-Within-Clay, not pictured.)

Update: Though the cause is unclear, it looks like Carver had somehow been added to the campaign PC list but not the world PC list. Or possibly he'd been dropped from the world PC list without apparent cause. Kicking him from the campaign and reinviting him only added him back to the campaign, not the world, so it didn't fix the problem.   We were able to fix the problem by going to the world PC page and giving the player the link from that page, which did successfully add Carver to the world PC list. This also worked for another character who was having the same issue. There must be something broken in the linking between world PCs and campaign PCs.