I started a new game to test - same world starlight shores , set each seasons to lowest 3 days , gamespeed 3 with camera on the park pond.
The pond start to freeze almost immediately when winter arrive , completely frozen skate-able when dropped around -4/-5°C
On the 2nd day it rains and temperature climb up to peak 9°C the pond was unfrozen fish-able in winter.
3rd day - snowing and temperature went down , the pond was frozen again. It remain frozen all the way till 2nd or 3rd day of spring.
9°C seems to be the key temperature for frozen pond to start melting.
I did not use cheats to adjust the temperature (didn't know there cheats for this) , i did adjust per season days from 5 to 7 in game once in my glitched game.
Going to shift my current playing family Sims to a new neighborhood world and see if this bug happens again ... hopefully not.
I'm not really into fishing aspect in Sims3 but sure is annoying staring at a frozen pond during hot summer