If by "highlighted bar" you mean highlighting the selected item then it works for me 1.38 on Mint
09:30:23: Geany INFO : Geany 1.38 (git >= c8c8371e3), en_AU.UTF-8 09:30:23: Geany INFO : GTK 3.24.20, GLib 2.64.6
The autocomplete list is a normal GTK treeview and the selection is normal GTK select, so its style is set by GTK. Perhaps your Arch updated the GTK theme you are using so highlighting is not visible.