Just to clarify again, this is not a technical bug.
Where the Coach assigns you an exercise like this, it wants you to do so with a single dumbbell of the weight assigned.
As Santiago previously replied, I try to give some explanation to this in this topic here 
I also say in the above conversation that I think we’re aware it’s not quite as intuitive as we hoped it would be, and I’m sure we’ll do some work to review this in the future.
