Just logged into the Forum for the first time in a long time to report this same bug. I only noticed it within the last week, thought "weird" and reset my city's DA to the correct ones but just noticed it has happened again twice in the past week including today.
The first time units had been deselected and I only had 4-5 of the original 8 units in the city DA. However, the last two times the correct units have been switched to other less favourable units. Not all the correct DA units have been switched each time and they aren't always switched to the same type of units; some have been replaced by Rogues and some by other units.
I can also confirm that the original (correct) DA units have been switched and NOT deleted; I know this as my correct DA units are from a higher Era and they are still in my fighting pool but have been deselected from city DA duties.