Engine too cool with heat selected

My brother has an '03 4.3 base model Blazer with manual AC controls. He's only owned it for a few months and has had this issue since he bought it. When the temp selector is set to hot, the engine temp indication gradually falls from normal far enough to turn the SES light on for low coolant temp. He has replaced the engine temp sending unit twice and it hasn't changed anything. The "dealer" he got the truck from has had his mechanic look at it several times and hasn't a clue. If he turns it back to the cold side, the temp gradually returns to normal. At first I thought maybe this was a million in one problem since the circuits don't seem to have anything in common except the grounds in the ECM. But then my son just bought an '03 Blazer Extreme about three weeks ago with manual AC controls and we just discovered it has the exact same issue. Sitting still it doesn't seem to happen as I tried that yesterday. Since these are the only two '03s I've seen up close and both have this problem, I figure they can't be the only two and I'm hoping someone here has seen this before. Coolant levels in the radiator as well as the overflow reservoir are good.

Bad T-stats?

I guess it could be. Mulling it over the last day, that's the only thing that remotely makes any sense. Maybe partially stuck open? Given what I've read here in the forum, maybe I should pick up four or five just to be sure I get a good one.

I guess it could be. Mulling it over the last day, that's the only thing that remotely makes any sense. Maybe partially stuck open? Given what I've read here in the forum, maybe I should pick up four or five just to be sure I get a good one.
That is exactly where I would start. T-Stats are only $5 - $10 bux. I'll bet they're stuck open.