There are explanations to both your problems.
I'm assuming the "tele" you're referring to is your 150-500. I would also hazard a guess that you're at the very end of that (ie 500mm). The lens likely has a maximum aperture of f/5 at the point (which is odd: it's likely f/5.6). This explains why you cant get any lower than that with that lens: its a physical limitation of the lens.
When your camera automatically stops down to f/13 (which, again, is very low), it's because of the teleconverter. A 2x converter adds 2 stops to the maximum aperture: so f/5.6 goes to f/8. Why it's going to f/13 is beyond me, but if you've stopped down to f/8 before adding the teleconverter, it could be doing the math by itself. f/8 + 1 = f/11, and f/11 +1 = f/16.
Hopefully this answers your question.
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