Sorry if this is a duplicate, I've looked through the reports and didn't see one for this. When I upgraded my world from 20w17a directly to 20w21a structures lost their bounding boxes. For instance, I had a witch farm, but when I upgraded to 20w21a I was only getting general mobs out of the farm, which did not happen previously. Additionally, I went to an ocean monument that I had already previously discovered/loaded and there were no Guardians spawning. I then checked my guardian farm at another ocean monument and there were no guardians spawning there either. My only conclusion is that the bounding boxes of these structures must have been deleted, but they should not have been because they were in chunks I had already explored.
When I upgraded to 20w21a I optimized the world and erased cached data to deal with the bug in MC-179845. I wondered if for some reason this caused the issue, so I restored the backup of my world and tried upgrading to 20w21a without optimizing the world and the bug still occurs.
I've tried to attach my world backup, but even in a zip file it's larger than the 10MB limit. I'm not sure if there is a specific file in the world folder that will give you the information you need.
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Thank you for your report!
However, this issue is a Duplicate of MC-184740.
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Yeah, this is a duplicate... Look at bug report MC-184740 . In the new 20w21a ALL structures got "renamed". Their names changed for example from "Monument" to "monument" but upgrading to newer version does not update the world files, so the structures won't work. This happens with every structure (Nether fortress, monument, witch huts and others.)