It appears that simulated left&right 8Direction controls are ignoring deceleration/acceleration settings. I don't have the time right now to make a minimal project so I can put in a bug. (Maybe in a couple of days I will.)
The work around is to copy the "Simulate 8Direction pressing Left" or "Right" and paste it below the copied action. Then for it to actually do the simulated action twice, you need to throw a 0.025 second wait in between them. (Don't try using a ".025" second wait, because the dialog can't handle numbers starting with "." instead of "0."