Air conditioner problems on 93

A few weeks ago, the AC compressor seized on my '93 Corvette. I guess 18 years is all they have in them. I took it to a trusted repair shop, and
had them replace the necessary parts, and upgrade the AC system to use to the latest coolant.
After getting it back, I found that there is a new problem. If I turn the climate control all the way down to 60 degrees, it blows very cold. If I turn it all the way up to 90 degrees, it blows hot. But anywhere in the middle, and it doesn't seem to regulate the temperature at all. It blows air that is neither heated nor cooled.
They have the car back to figure it out, but haven't come up with a solution yet. Anyone familiar enough to offer a suggestion?
Thank you.

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