There may have been monitors that detected structural issues in the hull (nucleation above), but at the pressures that the Titan was operating at, the amount of advanced warning is likely very, short--_maybe_ a second or two, if that. Probably not enough time to notice and reduce the pressure on the hull in any meaningful way.
The slow-motion porthole cracking you see in movies like "The Abyss" is a fiction created by Hollywood for dramatic effect. The reality is far more likely to be like water freezing in a pipe: everything is fine until it isn't.
The pressure the pipe exerts on the water inside of it keeps the water in liquid form until the temperature drops below the temperature/pressure equilibrium (~10F/-12C for residential copper pipe in the US). At that moment, the super-cooled water transitions from liquid to solid virtually instantaneously. <poof>.
The pipe analogy is the opposite of what would happen in the submersible: the pipe explodes due to internal pressure, instead of implodes due to external pressure. But the pressure going from sustainable to unsustainable in very short order is the same.
http://o-si-sec.com/2018/10/11/5-facts-to-know-about-stop-dr...
Water pressure increases by 1 Bar every 32ft (10m) of depth. To see the equivalent change in pressure in water (0.6 bar) as you see going up to 30K in the air, you only have to go down ~20ft (6m). The pressure on the submersible at 9000ft (2750m) is 276 Bar.
F16s and other planes with composite body panels will sometimes lose a panel due to water buildup between layers of the composite. The water freezes between the layers popping the panel loose from the frame or retainers holding it in place. That is usually not a life-threatening problem (though not desirable). A crack in a submersible pressure vessel at more than a couple hundred feet is usually fatal.
The description of the Byford Dolphin accident in ~300ft (~90m) of water or 9 Bar pressure, is illustrative: https://www.iflscience.com/byford-dolphin-accident-how-livin...
The pressure on the Titan would have been ~30 times greater than what occurred on the Dolphin.
This is an exaggeration, small cracks can develop and grow very slowly over the course of years. There’s lots of types and sizes of cracks.
[0]https://leehamnews.com/2019/09/30/boeings-737-in-another-pic...