Horse losing muscle daily

Product: The Sims 4
Platform:PC
Which language are you playing the game in? English
How often does the bug occur? Every time (100%)
What is your current game version number? 1.99.264
What expansions, game packs, and stuff packs do you have installed? Everything
Steps: How can we find the bug ourselves? Check horse muscle slider in CAS daily.
What happens when the bug occurs? Horse loses muscle everyday
What do you expect to see? CAS settings to stay locked unless the horses are coded like sims and not pets, in which case sims will still only lose muscle up to their creation level setting
Have you installed any customization with the game, e.g. Custom Content or Mods? Not now. I've removed them.
Did this issue appear after a specific patch or change you made to your system? No

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Not sure if its a bug or not but my horse is constantly losing muscle. Everyday the slider in CAS shows like a 5-10% decrease, continuing until it hits 0. Ive tried disabling its needs, not going for rides, constant feeding, which also doesnt increase its fatness, and different training types in case they had an effect but no dice. I thought maybe changing its body type to the one that starts with the highest muscle mass would stop it from going lower than the default but it still decreases. I use MCCC and have it set to maintain physiques for the sims but there are no options or flags for animals. My dog in the same household doesnt seem to have the problem and stays the same.

EDIT: I have tested removing MCCC and the horse still loses muscle. I Tested not riding the horse and that made no change. I downloaded RoBurkys Fitness Control to see if that would have an effect but the bug still occurs. Ive just morphed the horse to look like it has max muscle and set Physique to 0.