It is very difficult to say anything specific without more information. Did you run the 12 simulations at the same time, e.g., with the -multidir option? Otherwise it would be improbable that all simulations stopped at exactly the same time.
Did you get any output to the terminal explaining why they stopped? Have you checked the end of the log files? Did you run out of disk space? Were they run on your own local computer or on a remote machine with a queueing system?
Since the jobs ran for 500 ns I don’t think the mdp parameters are the cause. With the available information I can’t say for sure whether the problem was caused by GROMACS or some temporary problems with the hardware/software stack of the server. But we’re of course interested if others have experienced the same thing. It’s not impossible that it is related to PBC wrapping hangs after a system explodes (#4766) · Issues · GROMACS / GROMACS · GitLab, but as I said above, if they had already run for 500 ns I don’t think that’s the problem.