Another low cost test option would be to repeat the indoor test, but with a cable the same length as the one that may be giving you trouble.
If it's cable length, the IR will fail the same test that it passes with the short cable. If it still works indoors with the long cable, that indicates it's the outdoor lights.
Alternately, if you cover the camera with something light-proof where it is (like a towel or similar), that should make it dark enough to switch, and you can see the IR come on by watching the viewer.
This is something Dahua needs to add to their firmware - IR switching sensitivity adjustment.