An ocean monument spawned AFTER chunks have been loaded in earlier update

So I was looking at a very old level of minecraft, that has existed since probably 1.3. It is a level of an enormous railway that goes more than 5000 blocks... if you were travelling in a straight line! This track goes through caves, mineshafts, a stronghold, in the sky, to a mushroom island biome (and pretty much every other biome), through jungle temples, desert temples, above and through oceans... you get the point. Huge.

Anyways I came across a section of tract that did cross the surface of an ocean, and to my surprise, an ocean temple had spawned and replaced a section of my track, as well as the surrounding ocean bed. It even revealed an abandoned mineshaft directly below it. (Thankfully, not too much damage was done as it was only a straight section).

This ocean monument spawned after those chunks had been loaded in an earlier update, before deep oceans even existed.

Steps to reproduce? Ummm... I have no idea how or why it happened. I do know that I was here first in an earlier update and as cool as this is, it shouldn't have showed up.

In the screenshots, notice how the powered rail extends across 2 blocks of water before ending, and the light from the redstone torches remain.