Synopsis:

So, I started a new Minecraft (Bedrock-XBOX) map to complete all the achievements etc. on this version as I have completed majority of achievements on all others. I was about my third day in and had obtained all the necessary equipment etc. to finalize "The Deep End" achievement in which you slay an "Elder Guardian" I arrived at a Monument where I had seen and been affected by the "Elder Guardian". 

I saved not long after arriving at the monument because I needed to get my nephew, then later arrived home and my nephew (4-5Years old) then wanted to play but hasn't learnt to play with Hostile Mobiles(MOBS) yet, So I changed the game-difficulty from Hard to Peaceful and while he was playing I built a structure with easier access into the structure. Soon enough 20 to 30 minutes later my nephew went down for a nap and gave me about 40minutes to complete the achievement upon changing the difficulty back to hard, the "Elder Guardian" was not anywhere to be found prompting me to leave and return and still the guardian had not respawned/reappeared anywhere inside or outside the monument.

Assessment of Bug:

1. Location of an Ocean Monument with Elder Guardian present with difficulty on Normal-Hard.

2. Change of Game-Difficulty to Peaceful and then return to Game-Difficulty to Hard.

3. Elder Guardian fails to respawn/reappear.