Wet Joints on Flagstone Patio

We had a flagstone patio installed about a month ago - in late December concrete slab with the stones mortared on top. For the most part, we are happy with how it looks and functions. However, about a week ago we had a rather misty/rainy day over half and inch of rain in one day. About 48 hours after the rain it had been clear for that entire time we noticed the joints around a few stones appeared to still be damp. The rest of the patio looked dry. There were no noticable puddles that occurred during the rain, and the patio appears to drain properly it's leveled away from the house - I checked it myself. The picture attached is taken from our back door. At this time of year, that area of the patio doesn't get much sun, perhaps an hour or two a day. Once spring hits, we'll start getting more sun out there. The same thing happened when we got some more rain yesterday.

What do you think may be causing this? Is it anything to worry about? Thank you.

By the way, I like your website. I did not notice this type question being asked before.