Reordering the lemmings (in the level file) does affect this. Some orders raise an entirely different bug. It also appears that the bug occurs (with the same result) when the splitter is left-facing too; however, it only occurs if the splitter actually turns the lemming around (in either direction).
(Z = zombie, L = left blocker, R = right blocker)
ZLR - This one raises a bug where R ends up as a walker rather than a blocker when gameplay starts (however, he's still a blocker on the preview screen). This topic's bug does not occur.
ZRL - This topic's bug occurs, L becomes infected.
LZR - This one also raises the ZLR bug. This topic's bug also occurs, L becomes infected.
RZL - This topic's bug occurs, R becomes infected.
LRZ - The ZLR bug occurs; this topic's bug does not.
RLZ - This topic's bug occurs, L becomes infected.