Weird rotting issue with Anubia

Waking up this morning I saw a dozen leaves of one of my Anubia Nana leaves floating around my tank. At first, I thought it was my Oscar's doing until I looked at the mother plant, the entire rhizome has basically rotted into a mushy pulp, the leaves where fine but everything else basically turned into a mush like substance. All my other Anubia seem to be totally fine...

Has anyone encountered this before?

 

I did once, in my earlier fishkeeping days, when I buried the rhizome in the substrate. If it was not buried, no idea, sorry.

 

It was attached to a piece of driftwood, and the plant itself was about a year old.

 

I've read that excessive growth of algae on them can also be a cause for this to happen, maybe some sort of fungus that got on it? Not my area so was just wondering as saw this on the web. some also claim using too much excell can also cause this.

 

https://www.aquariumcoop.com/blogs/aquarium/anubias-rot