[BoarGules]

Nailed it.

The prehook function needs to be:

function OnGuildIdChanged(self)
    local guild_id = self.guildId
    ...etc...
    end

For some reason the function GetGuildId() always returns 1 in this context. And GUILD_SHARED_INFO.guildId is always one behind, because the prehook function gets called before the handler for EVENT_GUILD_DATA_LOADED in the GuildSharedInfo API class, and that handler is what changes the value in GUILD_SHARED_INFO.

Thanks for pointing me in the right direction.