As far as I understand this is affecting lots and lots of people, if you have a world that has been updated to 1.16 but was made before, then you remove a structure (as would be done with guardian farms which is the way in which it has affected me) then if you update to 1.16 those structures for some reason are no longer being counted as structures, in my case no guardians are spawning at all as it doesn't recognize it as a structure, I'm not sure why this happens but I do know that many people have been affected too, as you may see in my image no guardians are spawning, if you would like to recreate the issue, make a world in 1.15 or 1.14 (my seed is 8233760587623457723) and go to a structure (tp to -400 80 -1900 to go to the ocean monument that i used) and then remove the structure, then update to 1.16 and you'll see that the structure no longer produces guardians or whatever it normally spawns.