3dsmax- Modifiers are not exported

I’ve been trying to flip a model using the mirror tool.
The animation was fine, but the bones are messed up. But I found that by using the mirror modifier tool, I could put them in their normal orientations.
However, when I exported and reimported them, no changes had been made.
I tried collapsing the stack, this had no effect.

If it helps, I also looked in the .smd file for it: it has the list of the bones, as well as their rotations (I think). However, I saw no mention of anything to do with mirroring in the file.

Does anyone have an idea about what to do?