I loaded a backup of my world in snapshot 18w11a and my guardian farm was not working. The farm is located where an Ocean Monument was at one time, that I mined up along with a 75x75 block area with all water cleared out, and now only has 3 block water deep source blocks @ y56-59 for the guardians to spawn. In the snapshot no guardians would spawn at all. This farm has worked from v1.10-v1.12.2. Thinking maybe the spawn depth for guardians has changed I flooded the entire area (75x75 blocks) y60 down to y22 and waited. No guardians spawned at all at any depth.
So I checked other areas of my world where 3 other Ocean Monuments should be located and found no monuments in the area and also found nothing using locate. At this point I thought maybe it is due to my world being generated in 1.8, things change so I created a new creative world and manually searched for an Ocean monument as well as using MineAtlas. I could not find any and locate function never found any.

Update: I was able to locate a new monument in 1.12.2. Loaded an old backup, switched to snapshot 18w11a and went to the same location, Monument spawned, Elder guardians spawned, no guardians spawned. As soon as the crashes stop I will post a screenshot.