Description
In the examples below I show that unicode characters (the alpha) seem to interfere badly with syntax highlighting. It seems like, vim-pandoc-syntax struggles with combinations like $x^{a}_{b}$, when x is a unicode character. Interestingly, reversing the order, $x_{b}^{a}$ seems to work. Also, when x is just an ASCII character, it seems to work as well.
In the example, the last line will break all successive highlighting. In vim in particular, the effect is the last _ gets concealed and the i italicized.
Ascii works: $a^i_i$.
Ascii with braced superscript works: $a^{i}_i$.
Ascii with braced subscript works: $a^i_{i}$.
Ascii with braced scripts works: $a^{i}_{i}$.
Unicode works: $α^i_i$.
Unicode with subscript first works: $α_i^i$.
Unicode with subscript first and braced subscript works: $α_{i}^i$.
Unicode with subscript first and braced superscript works: $α_i^{i}$.
Unicode with subscript first and braced scripts works: $α_{i}^{i}$.
Unicode with braced superscript breaks: $α^{i}_i$.
EDIT: I have experimented some more, and to be honest, I am confused what is causing this. It doesn't look like braces affect this. It seems to be an interaction with unicode, but I cannot always reproduce this. When I change this:
$a^i_i$
$a^{i}_i$
$a^i_{i}$
$a^{i}_{i}$
to (notice the alpha):
$α^i_i$
$a^{i}_i$
$a^i_{i}$
$a^{i}_{i}$
I notice that all syntax highlighting is broken. In particular, on my vim, all the ^ in all $a^ are highlighted white, whereas with α^ the ^ will not be highlighted and all successive lines break.