Slitherine

In DLC 40, after finishing a scenario, I was awarded an SE infantry unit....but noticed something funny. I had only 7 infantry units when I ended the mission and ended with 7 at the start of the next one, including the SE reward unit. All 7 were on average 2-3 stars. So it was easy to notice that the SE unit REPLACED an existing 3 star core infantry?!

I lost an experienced unit for a green SE??

Luckily I was able to reload my last save and finished the mission with no reward and sure enough...I had my 7 2-3 star infantry units all intact.

I was under the impression that SE units were in addition to my core...not to replace them?

Is this a bug?

SE units does not replace existing ones.

In your case there may one of the following scenarios:

1) You have lost one of the experienced units in the previous scenario
2) In the new scenario you have deployed the maximum number of units (including the SE unit) and you are not allowed to deploy the remaining experienced infantry unit

I would wager you lost a "core" infantry unit in the allies final turn, then. Remember the Allies play a combat turn between when you hit "end turn" and the victory screen comes up. Losses are not shown automatically. I had this happen once or twice.

And to address why it looked the way it did (SE replacing core unit): If you reload, it's entirely possible the re-play did not eliminate the core unit. Getting the SE bonus unit is a separate function, and low-probability. So likely, you re-played, no loss, and then did not get an SE bonus. Getting an SE unit never removes a core unit; it sounds like a coincidence given the combat and award variables. A lot of play on GC '40 so I doubt it's a bug, more likely just the "roll of the dice" variability.