It's something on your end....mine works fine and I'm on 4.0.5. Look through the settings again. Make sure you don't have an override that puts the preset you're on to Series instead of Parallel (must be parallel for trails to work...you probably already know that, but I'm just checking).
Try setting trails to Off and to Series, then changing back again. Or try "writing the settings" to the PBC instead of just writing the "changes". Sometimes it does quirky stuff, and I've found that occasionally I have to write settings to it for it to clear back up.