How do I book a 2nd class seat with a 1st class global pass?

I am a holder of a 1st class global pass. Unfortunately all 1st class seats are fully booked. Now
I would like to book a 2nd class seat.

On Eurostar trains, the system shows that reservations are available for a 2n class price. But when I click, it shows no seats available. On dates where 1st class is available, I can chose both

I guess, the system shows only options for 1st class. How can I tune that I also can book 2nd class when 1st class is not available on this train.

(Yes, I know that I can sit in 2nd class with a first class ticket). I just can’t figure it out in the online reservation tool