The One pen to draw them all advancement is given to the player as soon as anything enters their inventory the first time.

Hi NillerMed, oky let me have a look at this (the first advancement I ever made, yay) - it worked fine on my SMP server strangely.
Was this in on a new world (e.g. your setup) or could people have already had the pen in the inventory before, so that the advancement triggered because it was added later? Cheers,-

Okay, MC1.16 somehow has a problem with that advancement JSON, I presume the advancement is anyway important to a "major minority", so I simply removed it for the back port. The fix version is redstonepen-1.16.5-1.0.8-b3. It's already in the approval process on Curse.
Thanks again for the hint! Cheers,-

Thank you kindly :D