For anyone else wondering what the warship was doing all the way over in Sri Lank, it seems there have been some military exercises that Iran took part in India, hence it was moving through the Indian ocean where.
Ofc, this hasn't been confirmed yet, nor have they confirmed whose submarine attacked the warship (although we can make some fair assumptions)

They haven't confirmed there was a submarine attack at all.

Sources in ​Sri Lanka's navy and defence ministry said the vessel ⁠had been attacked by a submarine and at least 101 ​people were missing in the incident that took place off ​Sri Lanka's Indian Ocean coast.

that's the best confirmation that I saw, but I guess we'll see more information later on