This is similar to a bug reported in PTR. So far on Disc Priest seems to only affect Power Word Shield. I haven’t had time to check other specs or do a detailed check on all spells in this spec, so it may affect more than this.
NOTE: turning off auto-self cast has no effect on this behavior. PWS also still only self casts even when friendly eligible party member in range is targeted but cast with mouseover or click cast option. The only way to PWS another player is to hard target them and directly cast it at them.
Tested using default interface no addons, using multiple iterations mouseover macros, PWS casts on self instead of the character or unit frame moused over.
Using click cast option in game, most spells seem to work as intended with unit frames, but PWS still only self casts.
Using clique, healbot, and vuhdo the behavior is the same, PWS only self casts.
EDIT: Self cast modifier SHIFT is selected in the game menu by default, so any click casts using this modifier will end up self casting until you go into the game options menu and change it or turn it off.