Roscosmos has issued a statement on the cause of Luna 25’s crash into the Moon on 2023-08-19 in a post on Telegram (in Russian). Here is a Google Translate rendering into English.
“Luna-25”: preliminary results of work to determine the reasons for the abnormal functioning of the station
On August 19, when issuing a correction pulse to transfer the spacecraft from a circular lunar orbit to an elliptical pre-landing orbit, the Luna-25 propulsion system operated for 127 seconds instead of the planned 84 seconds. As a result, the station switched to an undesigned open orbit and collided with the lunar surface.
It was established that the most likely cause of the accident was the abnormal functioning of the on-board control complex, associated with the failure to turn on the accelerometer unit in the BIUS-L device (angular velocity measurement unit) due to the possible entry into one data array of commands with different priorities for their execution by the device. In this case, the distribution of commands in data arrays is random (probabilistic) in nature.
In this regard, the on-board control complex received zero signals from the accelerometers of the BIUS-L device. This did not allow, when issuing a corrective pulse, to record the moment the required speed was reached and to timely turn off the spacecraft propulsion system, as a result of which its shutdown occurred according to a temporary setting.
Recommendations for additional activities for subsequent lunar missions have been formed.
Apparently, due to the command to turn on the accelerometer’s having too low a priority on the spacecraft’s internal network, it was never switched on and thus never sent the signal to the engine to shut down when the commanded delta-v had been achieved. The engine continued to burn for 127 seconds instead of the intended 84, lowering the periapsis on the other side of the orbit below the lunar surface, resulting in unintended lithobraking when the orbiter arrived there.