Put the summary of the error you have here
What I expected to happen was ...: "
That the shulker box kept my things in the base where you appear in the end that is obsidian
What really happened was ...: When I entered the end my shulker box was lost and I cut off the water I had as if I had reset that area
I have put a shulker box in the obsidian base that is generated when you enter the end and in that shulker box I kept many important things in what I built something and I left the end and when I returned it was in less than 4 minutes the shulker box had disappeared along with my things and I had put a bucket of water, I noticed that that bucket of water every time I entered if I left it on the water disappeared and I think that area or something like that is reset
Steps to reproduce:
1. ... Put the shulker box on the obsidian base on the end or a bucket of water
2. ... get the end and come back
it is not a bug, it is actually an intended mechanic (as far as I know in Java) where any block placed around the platform would will be deleted whenever someone enters the end. This is to prevent any endstone generating on the platform and potentially suffocating the player when entering the demension. Hope this helps 🙂