Hi
Its not a bug!
It means you cant move that stack to the red regions because there is enemy units nearby extending a ZOC (zone of control) on your stack.
which stacks you can move depends of the strength of your stack, the strength of enemy stacks the type of units (cavalry can make raids deep on enemy territory), if there are forts and fortresses...
Also you have and icon near the "envelope" icon which explains it.
This rule makes fronts less fluid and much more realistic. You can't slip pass big armies or fortifications unless you use a bigger force or cavalry and irregular units.
Look on the manual, it explains it much better than me
Its on page 29!
Cheers