I don't think it's RNG screwery only. I think levels may have something to do with it.
If you're on chapter 6 the enemies are close to level 13, correct? I think you spread your exp too thin.
Your characters were probably hit by a gambit that stops movement. Gambits can hit in large areas, not only the main target.
Also, Lorenz and Marianne are not naturally fast characters. My Lorenz has 14 speed at level 20.