Okay, so existing behaviour was as described (more-recently spawned lemmings appear in front) with one exception - the "selected" lemming is drawn furthest to front no matter what.
For future updates (V12.7.X updates included, if the code backports to it nicely, which I don't see any reason why it shouldn't), there are a few factors in determining priority. From furthest-to-front to furthest-to-back:
1. Selected lemming
2. Highlit lemming
3. Athletes
4. Regular lemmings
5. Athlete neutrals
6. Regular neutrals
7. Athlete zombies
8. Regular zombies
For the purpose of this, a zombie-neutral counts as a zombie, not as a neutral or as both.
(Before I release any update with this - any thought on whether we should have a priority rule between walkers vs other actions, or any other feedback on the above proposal?)