MTS don't eat plants, that is one of the reasons they are favored by planted tank enthusiasts, because they don't eat plants, stir up sand bottoms, & eat excess food.
If I had to guess, based on what you're saying "All of my plants are thriving except my red flame sword plant" I'd say that it's some sort of nutrient deficiency that is the culprit. If it was the MTS causing the problem then your other plants would show signs of predation too.
I have a planted invert tank stocked with both MTS & briggs as well as shrimp & have never had a problem with my snails eating my plants. Sword plants generally are pretty undemanding but it could be lacking some light or maybe iron.
If you can provide pics it would be easier for others to chime in & opine.