Tanner is again looking at coaching decisions in a vacuum. While Heiskanen-Klingberg appears to be a great pairing...they can't play 60 minutes a night.
The Stars were counting on Lindell to anchor a 2nd pairing but he was getting caved in. They tried Lindell-Polak for 300+ minutes and your precious Corsi rating was under 40. Lindell with Honka and Fedun wasn't much better. Super neat that Heiskanen-Klingberg had such a high Corsi...but how does that help the team win hockey games when your other defensive pairings get caved in every night?
Hmm, are you seeing beyond the vacuum yet?
So despite your suggestion to play Heiskanen-Klingberg, the Stars coach recognized that Lindell-Klingberg was the best way to shelter Lindell, which left Heiskanen to carry the pairing with Polak. I'm pretty sure coach Montgomery is well aware how great Heiskanen-Klingberg were together, but I guess decided he needed to spread the wealth to keep Lindell, Polak, Fedun, Lovejoy, etc... from getting killed every night. These are usually insights you gain from watching hockey games instead of spreadsheets.
Note that the Stars added Sekera this summer, perhaps hoping he can stabilize the 2nd and 3rd pairs to allow Heiskanen-Klingberg to play together more. Keep an eye on that.