Fixed Framerate and Substepping

I’ve been trying to implement consistent physics simulation and there are a couple of questions that I hope someone could answer.

Here’s how it went;
I set Fixed Framerate to 60, Max Substep Delta Time to 0.03333 and Max Substeps to 16.

Then I manually controlled actual FPS by giving more pressure to the engine(to be specific, I called ‘Draw Debug Sphere’).

The result was as I expected. Physics simulation was consistent(not completely, for sure) even if framerate went as low as 15.

After that I unchecked Use Fixed Framerate and went through the same tests. With substepping only, every simulation showed greatly different result.

And then I set Fixed Framerate to 60 again and Max Physics Delta Time to 0.03333 with no substepping. This time, surprisingly physics simulation showed consistent results just like the first experiment.

At this point I got confused. I do understand the relationship between rendering framerate and physics framerate.
However, isn’t max physics delta time(or max substep delta time) supposed to clamp delta time once the frame gets too low?

I mean, if max physics delta time is 0.0333, shouldn’t physics simulation show consistency as long as the actual framerate is kept below 30?

I think I’m missing something here; isn’t this the case where I should use substepping? If not, then would I just resort to using fixed framerate for consistent physics simulation?