I beleive your problem is firmware related, and The fat 32 file size limitation. Latest firmware fixes that problem other wise test with loop recording and you will se it records more than 20 min..
As omegamoto said loop recording will over write files, but will be a good test to asure you that your power bank is good enough and that the camera it self will record..
It will also be a good test to see that your memorycard is able to write bigger files over and over again without cluster size errors .