Ocean Monuments generated in older versions, do not spawn guardians

What I expected to happen was...:
Guardians to spawn within the bounding box of my monument farm

What actually happened was...:
They are not spawning and have not been doing so for a few snapshots now. Previously fortresses were in the same boat, but they are now behaving properly. The same snapshot where fortresses were fixed, this bug was reported resolved but is still present.

Steps to Reproduce:
1. Generate an ocean monument in a previous version of the game. (mine was generated in 1.13)
2. Load up the world in the 1.16 snapshots/pre-releases