From VikingSven

Sometimes on the first time opening the selling window, posting an item logs
[11:36:44] Auctionator: [] buyout at 999
to chat, but doesn't actually post the item. Subsequent items all do the same thing, and the only way I've found to make it work properly is to close the AH and try again

From plusmouse

The issue pops up when a full scan is running and the Selling tab is opened. Temporarily you could turn off the autoscan option - I'm looking for a better solution atm

Experimentation reveals that you need to reload the bag view twice to get it to work - even waiting 10s and reloading just once doesn't work, reloading twice within 0.01s does.