I had a VERY similar issue, except that there was NO hot water in all showers and baths.
Sims got blue and were about to die from bathing or showering in freezing water indoors, with thermostat set to warm , and radiators synched to thermostats. As soon as I took them out of baths or showers, they warmed up and were fine.
I do play with mods and CC, but I think the issue is related to FOR RENT build buy content.
Mostly for decoration, I had several off the grid water tank heaters (those for roofs and ground-I was only using about a dozen on ground). I had checked them when the issue happened first, but they had no options to fix, upgrade, turn on/off, or anything that I could have my sims do to improve them.
Trying to reduce the possibility of errors on the heaters, I deleted all but 2. The issue persisted, until I deleted ALL water tank heaters. After deleting the heaters, the options for using hot water in showers and baths were available again.
I think the build buy content from For Rent needs to be checked for bugs.