under_oom 0

That is your issue:

under_oom    0 or 1 (if 1, the memory cgroup is under OOM, tasks may
             be stopped.)

If set to 1, it means it's under oom control. Enabled.
If set to 0, like that it's not under oom control. Disabled.

You could watch the file /sys/fs/cgroup/memory/memory.oom_control, durring a stress test.

or

You could look at it's last modified date, to see if it was changed around the time of the last lockup. This will tell you if it was even atempting to do it's job.

under_oom 0

That is your issue:

under_oom    0 or 1 (if 1, the memory cgroup is under OOM, tasks may
             be stopped.)

If set to 1, it means it's under oom control. Enabled.
If set to 0, like that it's not under oom control. Disabled.