Unfortunately I believe they did know something was wrong. They texted to the support ship that they had a problem and were coming up, they also dumped ballast (an emergency method to surface). Their last moments were probably not pleasant, but mercifully the end came in 2 nanoseconds.
Supposedly this is the text conversation;
https://twitter.com/DGrootkarzijn/status/1675569914436788224?t=p5XKnoZeYKu8Wah-2ULhaQ&s=19