In main.jellyfin.bundle.js simply search for:
value:function(e){return void 0!==e?this.set("enableBackdrops",e.toString(),!1):(0,i.G4)(this.get("enableBackdrops",!1),!1)}},And replace it with:
value:function(e){return void 0!==e?this.set("enableBackdrops",e.toString(),!1):(0,i.G4)(this.get("enableBackdrops",!1),!0)}},Save the file and reload the cache on your clients to see your changes.
*** If you cannot find it (because of regex in search like in nano) try searching for enableBackdrops:function. This should give you the ability to find the string above ***
Source: BobHasNoSoul