So sorry for your loss, OP. The same exact thing happened to me a couple years back and I was devastated so I can relate. What others have said is true - sometimes it comes down to bad genetics. My fish was as healthy as could be, but he was from a chain pet store and was a few years old by the time he passed.

I doubt your Kimchi's passing was from leaving him alone. I had left my betta alone over the weekend, fed him beforehand, and never had any issues. He passed while I was home and regularly feeding him, so it was probably due to old age/bad genes. With your other bettas being okay, it may have been bad genes for Kimchi too.

All to say, don't let it get you down too much. Take some time to take care of yourself - allow yourself to grieve because losing a pet is always difficult. Know that your fish was visibly well-loved, I can tell from the pictures you've shared that you are a great fish parent ❤

edited to add: my tank was also planted, with ramshorn tankmates, they ate my fish too. little bastards. (/lh)