Land Rover Discovery 2001 - Heat is Cold?

I have a 2001 Land Rover Discovery. When I normally start my car in the morning to warm it up (I live in Minnesota) it takes about 5-10 minutes for the interior to heat up. I’ve noticed for the past few weeks that when I get in my car after 5-10 minutes of idle warm-up, it’s still freezing cold and the air is blowing out cold. Then, as soon as I accelerate, the air blows out warm and all is fine.

Just this morning I was stuck in stand-still traffic and had my foot on and off the break for about 3-5 minutes (with no acceleration), and the air got cold again. Then, when I was able to accelerate, the air blew out warm again…

Can you help explain what might be wrong?

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