I suspect they imploded.
These super deep subs are traditionally not reused very long, because the stress of the water pressing and then releasing weakens them. The more compression-decompression cycles they take the faster they degrade.
From all the reports, they got a lot of reports of issues that they ignored. I read that one of the reporters who saw it found it to be very jury rigged together. Apparently it was not certified in any way.
Even if they did survive and the ballast worked correctly, they would surface quickly (decompression sickness?) and cannot open the hatch from the inside. The thing doesn't float above the water, so its going to be a pain to find. Also they didn't paint it bright orange with blinking lights, its white, gray, and blue.
Overall, a lot of poor decisions and ignoring advice lead to disaster.