Sorry if this is a duplicate of issue MCPE-33992 for IOS or MCPE-21856 for Windows 10. I haven't found this listed for Bedrock on Xbox One. I noticed when I began walling off my Ocean Monument prior to draining that guardians were not spawning in the areas above the temple where I had erected sand walls. For instance, when I had the North and West walls completed to the water's surface, but the South and East sides of the temple were still open, guardians were ONLY spawning on the sides of the temple facing south and east (toward open ocean). This was back when they targeted fish, so if you stood on the platform in the center, you would see dead fish and bones only on those sides... literally nothing on the North and West sides.

Once the walls were completed and the existing guardians were killed, no more spawned. I thought maybe they were mob capped by guardians in the temple itself, but once everything was drained and I built my guardian farm, not one guardian has spawned. I've cleared out the caves below as best I can, I tried sitting on a platform well above the farm to take any of the caves I missed out of play, and I've even taken my Xbox One character to another area of the map and brought my Switch character to the farm in my Xbox world, and still nothing. I then rechecked when the 1.6 update came out yesterday, but they still aren't spawning. FYI I'm playing on Easy mode if it matters.

Not sure if it's related, but I've also noticed that there are almost no Magma Cubes in my Nether (same world). I saw one the first trip there and literally one other in dozens of hours spent there. Also, ghasts don't attack me there. Oddly, when I went to the same Nether but different location using my Switch character, I immediately saw two Magma Cubes, which I found to be odd.