Thanks HN folks for all the comments. To clarify a bit, the cables are pulled through PVC conduits under the flooring before being buried in cement. Currently the hypotheses for why the cable disintegrated so quickly is hydrolysis and paint solvents. Singapore is extremely humid but this doesn't explain why the exposed cabling on the other end is still healthy and not crumbly.

The second possibility is that I keep the leftover wall paints (Nippon Paint Vinilex 5000) in the same room and have noticed that much of the solvents have evaporated. It is possible that the solvents in the air might have caused the cable to fail in 3 years. It would explain why the other ends that aren't exposed to the air inside the bomb shelter aren't falling apart.

Some other learnings from this. Buried cabling should always be permanently fixed and attached to a patch panel instead of dangling in the open. That was the original plan but I figured it wouldn’t be an issue. I was wrong. Always measure exact length of buried fibre cabling as they aren’t meant to be stored in loops.