I think these issues often stem from a fundamental disconnect in the way the two partners see the family, from the beginning of the new relationship.
For the new partner, they decide at the outset that they can cope with the existence of previous children - they can cope with EOW, or even 50/50, if that's going to be the arrangement, and while the child is there, they will do everything right, make the child welcome, include them in the family - as long as they are able to make the non-contact time entirely theirs, and not have the stepchild or its mother intrude in any way on that time.
For the NRP, as for any loving parent, the optimum is 100% - we all want our children with us all the time, in an ideal world. The shared custody is a compromise, and a strain, and they miss the child all the time they aren't there. So things like sharing pictures, or facetiming, help enormously, and if the relationship with the ex is decent enough to facilitate that, so much the better.
Happens all the time on here - stepparents getting upset because the NRP wants to see more of the children in the holidays than they feel they agreed to, or the teenage stepchildren want to come over more often or ad hoc - this isn't what the stepparent agreed to, it's more contact than they wanted, but the NRP can only see more contact as a positive and is hurt that his partner doesn't. Or the stepchild hits 18 and the stepparent thinks "yippee, they're an adult now, we're off the hook", and of course it isn't like that at all - he's still their father.