I've encountered similar pattern over the past 2 days. Master Virusbuilders in missions heal FULLY.
The first mission had a room with 2 MVBs. It was evident that one was healing the other. It wasn't a gradual heal, but rather a full HP restore (from 25-33 to 100% health). I shrugged and moved on.
I've encountered the same problem last night while in SFH. This time it was a single mob. After 3 iterations of this I cast a nano shutdown which solved the problem.
Is this intentional?