Palpatine is hit by lightning that is reflected back by Mace’s saber. I think that might have something to do with it rather than something inherant to the lightning. Also, other than Ian being not a thin in the PT as he was in ROTJ, the ROTS makeup is pretty close to the ROTJ makeup, just done differently. It is just very hard to see anything other than his face in ROTJ, but there are occasional glimpses under the hood, enough to be sure the freaky forehead is there in ROTJ.
The make-up being not exactly the same is not really an issue (I know most fans despise this make-up but I don’t) since there is a 24 years gap between ROTS and ROTJ. The photography is not the same as well (in ROTS he’s in plain light half of the time). Only his “new” appearance in TESB is a problem (it’s only two shots, not the worst SE change to date).