Yes that is probably causing problems. If it’s not, it absolutely will in the future. Water will get in and damage the shield allowing interference in. When the water gets into the dielectric it causes issues with signal performance starting with the higher channels (skin effect). Your data channel is probably higher in the channel range so will be affected early on. Replace that cable or at least the part outside. Replace the splitter too just to be safe. The night day thing is probably connected to the sign level in general. When temps drop, signal level goes up slightly. It’s just easier for signals to pass in cold. You’re probably right on the edge because of the water effecting your signal so that slight fluctuation is making a difference for you. Could be that the water is freezing at night too but it’s all the same end result. Replace the cable. All that being said, you could have other issues but that is a great place to start.