As is usually the case in Thailand we'll probably never know what actually happened.
Bell 212 main rotor has a lot of inertia and is semi-rigid, so even with a dual engine failure rotor RPM won't decay as fast as say an eggbeater light training helo like a Robinso R22.
But reports of parts of the aircraft detaching could suggest a rotor separation.
In 2021 a 212 crashed in Canada after a retaining pin failure on the main rotor hub. That led to Bell issuing service bulletins and various aviation agencies worldwide (FAA, EASA etc) issuing Emergency ADs mandating compliance with the service bulletins.
On 05 July 2021, Bell Textron Inc. (Bell) issued 4 alert service bulletins covering model 204B, 205, 205B, and 212 helicopter fleets. The alerts required the review of the aircraft’s technical record to determine if any main rotor hub strap retaining pins with the serial number prefix FNFS were installed. If pins with the affected prefix were installed, they were to be replaced with pins of a different serial prefix before further flight.
https://www.tsb.gc.ca/eng/rapports-reports/aviation/2021/a21w0045/a21w0045.html