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I realize TXT2MTXT is an express tool, therefore officially unsupported but I just noticed this behavior and thought I'd mention it for those that are unaware.
Using Vanilla 2014 or 2015, I've not tested other versions:
If you have a string of TEXT that contains a % (percent) sign and convert it to MTEXT using TXT2MTXT, two additional percent signs are added in the resulting MTEXT entity.
[Apparently that's a version-specific issue -- it doesn't do that back in 2004, whether using a font .shx supplied with AutoCAD or a .ttf font that is not.]