Just solved it. Tried running #UndertaleYellow through #Proton and it works like a charm. Took a bit of preparation: finding #Fedora #FusionRPM Non-Free - #Steam repo in #KDE Discovery and installing that particular Steam, then running Steam from terminal (because default shortcut does jack shit) and forcing #SteamPlay #ProtonExperimental compatibility tool on the non-Steam game.