Overwatering can do long-term damage to rubber plant foliage (even just one overwatering).
I have a non-variegated rubber plant that was droopy and not as vibrant as it could be; it looked a lot like yours, sans the variegation. The solution to the coloring problem and lack of growth seemed to be to give it a houseplant dose of 24-8-16 All-purpose Miracle ...
Ouchy — That looks very much like sunburn! A thing I notice in the first pic: see how there is less brown on the understory leaf; and only on it’s edge? Looks as though the leaf above it provided some sun protection for part of the day.
Try pulling this guy into a bit more of a bit more shaded position.