The one battle spell mod command does not work on non commanders. I modded an archer with #onebattlespell "Eagle Eyes" and they didn't get the precision bonus. I made the same archer recruitable as a commander, and they did get the bonus.
That's not a bug. The #onebattlespell command is only supposed to work on commanders. IIRC, I believe KO mentioned this in the 'getting excited about the next patch' thread or something.