When using qutip, which has mathtext.py in the backend, after setting on an axis the label '$\left|-\right>$' the text is not shown

For the label '$\left|\minus\right>$' only a minus sign is shown. It could be that the \left \right symbols are too small to be seen.

The label '$\left|+\right>$' is shown without any problem, and any other symbol instead of -, also.