I wanted to hunt guardians with a new bow I had enchanted and I knew of an Ocean monument nearby. I boated over it and wandered around over the open penthouse structure. I did see the tail of an elder guardian and they cast the mining fatigue on me.  No regular guardian appeared. I had noticed earlier, in version 21w41a, that there seemed to be a lack of guardians on this same monument, but I did not actively try to provoke them. This world was originally created in 21w41a and continued in 21w42a. 

I tried searching for this same issue but only got hits from a year ago which were listed as resolved.