I've a theory that when you spectate a player, this somehow interferes with the new hitbox code.
If spectator is like commander mode, a comm is really just an invisible, invulnerable player flying thru the air. The bug where a comm looses his armor goes to prove this (not detailed due to exploit).
So, the hitbox of the spectator entity may be blocking the line of sight. That is, the code might be missing one of the arguements for LoS detection that has it ignore specific players or items [via one of the numberous "Trace" methods in the HL-SDK].