I know nothing about it and I was surprised to read about this situation because I would have assumed a submersible would be in fairly constant sonar contact with its surface vessel. I would've thought it would be followed a rigidly planned route and its location relative to the surface ship would be well understood at all times, i.e. "you are 3508.3m below us, 340m east, 233.1m south."
How does a submersible "get lost"?
The support vessel relies on acoustic pings to track the submersible. If the submersible stops responding, or is out of detection range, you'd have no idea where it is.
"Rigidly planned route" is not really the case either. You're not going to drop in at the exact same GPS coordinate, and there might be currents that push you.
Sure, but you'd know the last location, wouldn't you? Especially if it's pinging at least once a minute? And if the craft failed, surely they would just set down on the floor nearby, not travel miles in a random direction sideway.
I'm not an expert, but I think this is an incorrect assumption. We can navigate it decently on the surface (even then, plenty of ships sink/go missing each year [1]) but underwater is a whole different story.
> Why is this vessel unable to stay in contact with its parent
That is kind of the big question here.
[1] https://en.wikipedia.org/wiki/List_of_shipwrecks_in_2022