To reproduce: Play a level with 10 lemmings, 10 required (e.g. Fun 4).
The lemming count turns red as soon as one lemming is saved.
My understanding is that the lemming count should only turn red when there are not enough lemmings remaining to solve the level. I assume the engine is testing whether the remaining lemmings are less than the original save requirement, not taking into account that some lemmings have already been saved.
Your understanding of what should happen is correct, and your guess as to why this happens is the same as mine.
Fixed in commit 45daaba.