some mobs and items that I have interacted with disappeared

Minecraft has been very buggy lately, I do not know if it is a problem with my world specifically or generally. I was playing the other day and everything was fine, I saved the game as usual and turned the system off. The next day I came back in and all of the tropical fish in my aquarium and outside pond (which I had to transfer from a reef with buckets) had disappeared. My dog that I had named and tamed also was gone. I had a barn full of horses (including a skeleton horse) and donkeys, all of which were gone as well. Lastly, my armour stand with enchanted chain link armour and a looting 3 diamond sword were gone. This is very discouraging, as I put a lot of time and effort into building, creating, and finding those items and mobs. I really hope something can be done about this problem and I would be more than happy to provide my seed number.