Not super serious, but still a bit annoying. Basically, a custom formation with five doesn't appear to work correctly.

 

If you make a formation like this:

 

a  b  c

 x  y

 

You end up with this:

 

a      c

 x b y

 

The middle front rank guy ends up in the back.

They act weirdly from time to time, especially when new party members are added that were not accounted for when the formation was set up. Switching to another formation and then back helps fix the problem.