Missing Labels from Legend With Subplots

Question

I'm trying to plot multiple things in a figure with subplots. When I try generating a legend for the figure, it is only showing labels for the data in the very last subplot, instead of for the entire figure.

fig, axs = plt.subplots(3, 2, figsize=(16,18))

first_pops = {"M71 (Masseron)":[1., "purple"], \
              "6273 (Yong)":[0.1, "yellowgreen"], \
              "2419 (Carretta)":[0.0, "olive"], \
              "6218 (Carretta)":[0.25, "darkcyan"], 
             }

elements = ["O I", "Na I", "Mg I", "Si I", "Ti I", "Ca I"]
element_labels = ["[O/Fe]", "[Na/Fe]", "[Mg/Fe]", "[Si/Fe]", "[Ti/Fe]", "[Ca/Fe]"]

for k, ax in enumerate(axs.ravel()):
    element = elements[k]
    element_label = element_labels[k]
    
    for GC in first_pops.keys():
        
        try:
            plot_background_stars(glob_dict[GC]["[Fe/H]"],\
                                  glob_dict[GC][element_label],\
                                  label = GC, ax=ax, color = first_pops[GC][1],\
                                  s = 140, marker="*", facecolors='none', linewidth=2)
            
        except KeyError:
            print(GC+" doesn't have "+element+" measured")
            
    ax.set_xlabel("[Fe/H]")
    ax.set_ylabel(element_label)


handles, labels = fig.gca().get_legend_handles_labels()
by_label = dict(zip(labels, handles))

fig.legend(by_label.values(), by_label.keys())

I've tried replacing "fig" with "plt" and vice versa.

I'm using the following to plot each data set:

def plot_background_stars(x, y, ax=None, **plt_kwargs):
    if ax is None:
        ax = plt.gca()
    
    ax.scatter(x, y, **plt_kwargs)
    
    return(ax)

The resulting legend has just two of the plotted colors, instead of four. Like maybe it's only pulling the labels from the last subplot? But why would it do that?