Thanks for your reply 0xSoul!
Before I saw your post I had already got a hint about the jumper from this thread Argon One v2 used pins - #4 by pi-erwin. So I opened the Argon One case and moved the jumper inside and now my RPi4 boots automatically after power up as as you describe.
I have bought my Argon One case with the RPi4 already built in and the instruction manual that came with the case did not say anything about this jumper. This is something that Argon should include in the instruction manual if the current and future version of the Argon ONE case is supplied with this jumper.