additional backslash when exporting equations with subscript

[xibeisiber]

Hi, in my TeXmacs document I write an equation such as $\pi_3=\pi_1+\pi_2$, and export it as markdown file. the equation becomes $\pi \_{3} = \pi \_{1} + \pi \_{2}$.

There is additional backslash before subscript "_".. how can I get the right format?

I'm using TeXmacs 2.1.2 on ubuntu 20.04.

Thanks!