From the first picture, it is easy to see, that the bedding compound/ glue was not applied in a sufficient quantity. the differences where the teak touched it, and those where it did not, are clearly distinguishable. So there were voids beneath teak, and they represent a good path for water
Secondly, I see that you succeed in taking the staves off without breaking them. This is definitely the proof that the bedding compound did not do it's work
What is easy to conclude, is that if there was a water
intrusion in the area below the teak, it had a chance to spread all over. Even if it did not find her way into the core of the deck, it is just a question of time. Also, the water trapped beneath surface, will "boil" and make the problem develop faster.
This is the almost the same situation we have found on a Formosa
we restored completely. Check the sides of the deck. Are there any water residues coming out slowly, hours after the deck was washed down by rain or sea? (or hose). If so, the answer is simple, but not cheap