In 41.51 (no mods), when playing split screen co-op some zombies fail to appear on screen for both players.
Generally, if a zombie cannot be seen by one player, it can be seen by the other. I can't entirely prove it, but that's how it seems. Zombies will appear after having been invisible upon approaching to very close range (within melee attack range with a longer weapon) of the player.
Both player 1 and 2 can be affected.
Added a screenshot - 2 identical characters (no profession or perks) looking at the same door. P1 can see some door bashers, P2 cannot. Immediately prior to this, P1 could not see a zombie approaching from the letterboxes but P2 could.
Additional steps taken:
Uninstall via steam
Manually cleanup steam leftovers + workshop files.