I can confirm that. I have tested it with a couple of Routerboards within the RB4xx series (RB411AH, RB433AH, RB450G) and on all boards this bug apears. I used RouterOS 4.11. On an older Board RB532 the bug appears too. In the CLI-interface the confederation-peers are listed and the boards do certainly work within the confederation.
I have tested it now with RouterOS 4.11 on a Routerboard RB1100 (PPC) device. The bug in Winbox does NOT appear. Everything looks fine.