I checked the code, twice, but couldn't find anything which would make the ice weaker while the temperature is below freezing.
But if it is early winter / early spring, then it is possible that the ice is only barely safe, and any marginally small change can make the ice unsafe. Like, daytime temperature being above freezing for a few hours, and there being either full sunlight with no clouds, or heavy rainfall - both of which wear the ice faster than usual.
I'm afraid this is all I can say, reading the weather simulation code. Once Sami has time, he can check your savefile to see if it is more about the actual ice thickness, or more about the safety measures when estimating when the ice is safe to walk on.
And, just for the sake of clarity - did you try to walk on the exact same location / body of water than 3-4 days before? And before it was safe, but now it is no more?