This isn't a deficiency of LAN games, it's a deficiency of the observer mode Blizzard added to the game when they remastered it. The code they wrote to deal with stealthed units as an observer mistakenly changes the actual game state when you change player vision (i.e. set the game to only show one player's vision), which leads to the game state desyncing for the observer that does this. I run a project (ShieldBattery) where we've developed numerous patches to the game including a fix for this bug, but pro games don't use our stuff so they're stuck with the bug unless Blizzard decides to maintain the game again.