Invisible units have been an on and off thing since retail. You can often temporarily fix it in Campaign by doing a Quick Save and Quick Load, as that seems to sort of 'refresh' everything. Sometimes just LOS/visual detection for a Mech stops working, sometimes it's just the sensor detection, sometimes both, and once it starts happening, it usually gets worse as you go along. It's more obvious when you have isolated 'Mechs, as sometimes the LOS of other Mechs will pick up the slack, and thus you never notice that one Mech can't see units by itself anymore.
Possibly related: The northernmost sensor tower's sensor circle in Reacquisition: Base Gemini frequently disappears as you progress along the mission, though it may continue giving sensor data.
I seem to recall using a Minelayer in the mission where you take out the Liao chemical plant(using it to fill out the scattered minefield; I used to love letting the Catapults kill themselves by dragging them back through the minefield; also, it generally guaranteed they'd be salvageable) would often seem to cause the rest of the units to go invisible to my scout/sensor 'Mech.
I also noticed that pilot voices would often stop playing when I salvaged a lot of 'Mechs in a mission(if it's a memory issue or something similar, it could be related).
I used to suspect it might connected to having too many units visible/active at a time(dead unit hulks, salvaged Mechs and possibly mines), or perhaps leaving the game running for too long. Though I also used to partially blame my old computer, but it's since happened on my current computer.
As a separate mention, a couple of possibly other minor things I noticed specific to the current version: The voice announcing the arrival of the repair truck no longer ever plays, and when a repair truck is depleted, it no longer automatically 'dies', though it still can't do anything except move around.
