Definitely an issue with the stack.  When entering combat the game kills off random units (usually Relias first) so that you can only play 9 units (max) in the field.

Had this problem happen in that same battle (after getting Janusk, I had a 9-stack, and Relias died at the start of the next battle).

I only had 9 units in the stack (and without Relias, I started with only 8 on the battlefield -- I counted). So I think the bug is a bit more subtle.

In the campaign, it is not rare at all once you get far enough. It is pretty much guaranteed, since you are given both units and champions, and the champions cannot be killed or disbanded.

In the sandbox, it is rare; I have not run into it so far in any of the betas.