This is how Line of Sight works with temporary objects like ice blocks and is not a bug. You must turn off your auto attack if the Ice Block is close enough to the boss for you to melee, or it will not block LoS.
The original intent is to prevent situations where you’d get behind an iceblock in melee but still be able to hit the boss while in safety. As a result, if you are near an ice block and are attacking a creature, the iceblock won’t block LoS. This has functioned this way since Sapphiron from original World of Warcraft in 2006.
Hope that helps!