Went to inspect the topbar only to find a pile of dead bees and the comb being munched by wax moth.
The only satisfaction was feeding wax moth larvae to the chickens.
They didn't starve, there was still honey in the comb.
Probably virus damage as a result of varroa. The topbar is hard to treat. You can't close it up for Apivar to have the maximum effect, and you can't dribble oxalic acid because the bars are closed up.
It might be possible to fume them using an oxalic acid heater and crystals, but to be honest, I'm not going to bother finding out.
I think the topbar must find a new home.