Hi there! I do not know if you've got the answer to your question, but here is the way to fix the issue:
Click on the icon with an Army General portrait then click ''execution" button (2nd left button) and then choose "aggressive". Done. You can do it for selected army general or for the whole group of armies if you click Field Marshal portrait. Just double check if aggressive is "on" when you reassign generals/armies etc. Have a GG!