After some additional experimentation, I was able to finally remove the bug - by toggling off the event setting line of sight for enemies to be blocked by walls!
I have no idea why this should be the case, but it appears that the line of sight bug is somehow dependent on this event as well:
[code:1opfyid9]System: 1Start of layout
> EnemyAdd LOS obstacle Wall 0
As the bug occurs with only these two events, it probably doesn't involve any others.