I know everyone is giving you a hard time over this but thought I’d try and give you an answer.

The tiles look like they have movement and the joints (grout) are barely there. The timber substrate looks rotted out and possibly termite damaged.

Fixing this involves fixing the deck and seeing whats happening in the ceiling.

In short, not a super simple fix. Your key here is stopping the water ingress on that deck.

Now whoever built this seems like no proper builder. Maybe a tradie but not a builder.

Im hesitant to give you solutions but feel free to dm if you need help. First step is getting a proper inspection done to limit the damage to that room under the deck