The script that applies the selected item is mev_pause_items_use_actually_use, in that you'd want to remove this code at the end which destroys the current menu and the preceding one (the "use/throw away/hold" menu):
instance_destroy(daddy) instance_destroy()
I don't see the buggy behavior in the engine source file (ggui_menu_handle also has a check that only runs the left/right handler if you've allocated a menu that's 2 at least slots wide so it shouldn't even attempt to move the cursor in the monster menu which is fully vertical) so it might be something you've changed?