Here is a bug I have seen before. It happened my last PBEM as Reds, as a matter of fact. Latest steam version. Budenny "melded" into a unit in his stack during combat. Now the same thing has happened to Blucher in another PBEM. Here is a screenshot after a recent combat phase where a stack Blucher commanded (he had formed a corps) was attacked. Blucher disappeared at the start of the combat and apparently did not participate or lend his stats. When I keep the stack together, Blucher is no longer visible. Then I checked through the stack and found Blucher - or his portrait at least. He had "melted" into a Red militia unit (see screenshot). When you put this unit back into the stack again, he disappears and the stack appears to be uncommanded.
Any thoughts on this? I have asked my esteemed opponent to see if re-running the turn helps.