I finally figured out how to reproduce this one with Unclean Beast and how to workaround it.

1) Right-click another demi-god at a distance to move to and attack it.

2) While attacking, cast Ooze, which is instant cast (no bar) but will stop your character from moving.

2.5) At this point I usually cast his seld damage buff spell, which does have a casting time.  I'm unsure if this is required to reproduce.

3) Have opposing Demigod run away from you.

--> Notice that Unclean Beast doesnt' follow and no amount of right-clicking on target will cause him to move.

4) Right click on the ground to move to a spot, this will "unstuck" Unclean Beast.

5) Right click on target, you can now path to them again.