Automatic choice of curly and straight extensible braces in MTPro2

The MTProII fonts provide two sets of curly braces, the straight extensible ones and the curly desinged braces that extend up to 4in high…. and that is all very dandy, except that when using the {cases} construct from the amsmath it (automatically) sometimes chooses one type and sometimes choose another in a mix that does not look very nice:

documentclass{report}

usepackage{amsmath}
usepackage{mtpro2}
begin{document}

[
begin{cases}
a_n=2 a_{n-1}+1, quad forall,ngeq 2,
a_1=1.
end{cases}
]

[
begin{cases}
a_n=a_{n-1}+a_{n-2},quad forall,ngeq 3,
a_1=1,
a_2=1,
end{cases}
]

end{document}

Is this an error in the package that loads the fonts or something that could be controlled from the outside?

I have been looking at the code, but cannot quite locate the place where this is controlled.

Is it possible to handle this differently by using fontspec and avoiding the package mtpro2?

I am aware of the question/answer here, but that deal with a different problem — not the automatic choice of cases.