When I get these things its usually because one soldier is stuck behind a tree.

In the case of a routing unit, the routing unit has crossed the red line, except for one soldier somewhere on the battlefield, bouncing of a tree. Killing that soldier often resolves the issue.

In the case of reinforcements not having entered the battlefield, there is usually one soldier outside the red line stuck behind a tree, causing part of the unit not having entered the battle field. Nothing u can do about it really.

I actually had one reinforcement unit moving along the enemy wall, in range of all the towers. After the battle the unit was decimated, even though it hadn't entered battle