I'm guild master and I did some bids for my own guild, that worked fine.
Now, my friend also has a guild and gave me permission to bid on a guild trader.
He's away so he asked me to bid for his guild.
This is what happens when I go to any guild trader and select "Bid on Guild Trader":
It gives me a dropdown of 2 guilds (my own guild and my friend's guild).
As soon as I select my friend's guild from the dropdown, it will immediately switch back to my own guild.
Making it impossible to have my friend's guild selected and do the bidding for them.
I play on PS5/EU.