[HR] Sims reset when mounting a horse

I have a repro today, on a vanilla install only a few days old, that has never had any mods:

Product: The Sims 4
Platform:Mac (M3 Chip, OS 15.1)
Which language are you playing the game in? English
How often does the bug occur? Every time now, but did initially work. I then exited the game and came back a day or so later to find this error.
What is your current game version number? 1.111.102.1230
What expansions, game packs, and stuff packs do you have installed? All expansions up to and including LD | all game packs except MWS, JTB, SV | all stuff packs except CCSP, MFPSP, TSP,  BNSP, KSPS, MHSP | I only have the following kits: BDK, ILK, BNK, GGK | I also have HP installed

Steps: How can we find the bug ourselves? I haven't done extensive testing, so I only know that the issue occurs with the horse that I adopted into my family. But it reproed100% when trying to either mount or ride that horse. The sim and the horse are both still gaining horse riding, temperament, and endurance. I am not sure whether the issue began after a certain milestone was reached in those skills.

What happens when the bug occurs? Sim briefly mounts the horse before reappearing beside it. Also, immediately gives the chilled moodlet to a sim with the iceproof trait. Also, the colour of the horse's tail decoration had changed.

I attempted to reload the save again, but the issue initially persisted.

Subsequently exiting to the menu and reloading the save to do further testing with a different sim and the same horse showed that the new sim could mount the horse correctly. Once this had occurred, the original sim could mount the horse again and ride it. Subsequently, even reloading the save and not repeating the pattern with the alternative sim, the original sim was able to ride the horse again. So I'm unclear on whether reloading the save again cleared it up, or whether mounting the alternative sim helped. Though, given I didn't save after doing that, and then reloaded again to find I could mount the original sim successfully, I can't think how it would have been a factor. When the horse was rideable again, the colour of the tail decoration had returned to the originally selected one.

What do you expect to see? Sim successfully mounts the horse. Potentially rides it if that was the instruction.
Have you installed any customization with the game, e.g. Custom Content or Mods? No, never.
Did this issue appear after a specific patch or change you made to your system? Not that I am aware of. Horse was functional yesterday [January 3rd/4th (early morning)] and non functional today [January 4th evening]. Horse was then functional again following further testing in the course of writing this report, as discussed above.
Please describe the patch or change you made. No changes made to my knowledge.

Edited to add:

On closer observation, the tail decorations and bridle seem to change when the horse is mounted or un-mounted. When I was seeing the bugged state, the horse was displaying the tail decorations (I didn't notice the state of the bridle) that it now shows when properly mounted, but without a sim on its back. As I can no longer repro the bug, I can't tell if this happened only after an attempt to mount the horse, or if it loaded in that state.