If the overall performance was poor, and something so basic was clearly an issue, then maybe they don't realise?
This is like a development candidate being poor at simple interview problems - it simply is the case that often you don't know what you're doing wrong until you've developed skill, but people around you saying you're doing great can suppress your desire to work on that skill in favor of rushing ahead to interview where you fall flat on your face.
(An Open Mic D-Chord is a D-Chord (D A D F#) where they don't mute the lowest string and thus put a really nasty sounding E note on the bottom of the chord)
A constructive criticism isn't a one-liner on HN.
If you're skilled enough to show how that transition works, show it to them and make them learn it.
Which we, as a society, also seem reluctant to do. "oh you must not be good at that, try something else", instead of "if you want it you are going to have to work harder"
I'm changing the name of my band to this. ;-)
It's muscle memory, plain and simple. You just have to do it a lot, over and over. I've had music teachers essentially say "that part sucked, you need to spend more time practicing it". There's no trick to be told, there's no intellectualization that will help. Practice.
That doesn't sound like very constructive criticism.