I think you're probably right, the radiant heat is giving it warmer night temps.
Are you close to the bay?

One of the problems with growing tropicals and subtropicals in the bay area is the night time temps. We do get lots of hot days, but usually when the sun goes down the fog comes in, and the temps can drop quite a lot.
It's sure great for humans, nice for sleeping, but for citrus and tomatoes, it can slow them down I think.

Not that they suffer really, but we don't usually get that kind of fast growth in the summer like they do in the the interior valley or down south with their warm nights.

Also, it would radiate heat in the winter and probably help with those frosty nights. Good idea!

By the way, how do you like your Yuzu? I love mine! I have two. Not a ton of juice, but it is heavenly! And the zest is amazing. I made Yuzu-cha, which is like a marmalade that you spoon into a cup of hot water to make tea. Delicious on a cold night!

I don't know why they're not more popular with those in colder climates. They are very winter hardy. Supposedly they can survive down to 10*F. They bloom late too, so the flowers usually don't get frosted. Mine loses most of it's leaves in winter, and they pop right back with the spring. Wicked thorns though. I just snip the lower ones off on my dwarf tree.
Tough tree and nice fruit, you folks in marginal citrus areas really should try them. And the fruit is really expensive in the stores, if you can even find it! Totally worth growing!
(disclaimer; not my pics, found on-line)

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We're about 1/4 mile from the bay. It can get chilly at night but never really cold. The weather here is best described as 'mild.' I enjoy my Yuzu - it's got a unique aroma and flavor that really appeals to me. I can't wait until the tree starts producing in quantity.