Add_vrect not showing

i am trying to get a rectangle to be displayed in my plot and am having a few issues with its display. I have a subplot, with only 1 plot, but with a secondary axis showing a second dataset. I would like to show a rectangle on top of these plots showing the presence of a particular event. This rectangle doesnโ€™t appear to display.
My code is as follows:

#Create subplot with secondary axis
fig4 = make_subplots(specs=[[{"secondary_y": True}]])
#Add initial traces - NB. x and y values have been predefined from a pandas df
fig4.add_trace(go.Scattergl(x=SBAR_time_09, y=SBAR_Hz_Delta, mode='lines+markers', name='Eastings',line_color='magenta',connectgaps=False), secondary_y=False)
fig4.add_trace(go.Scattergl(x=SBAR_time_09, y=SBAR_FixT, name='Fix Type', line_color='red'), secondary_y=True)
#Add vrect
fig4.add_vrect(x0="12-09-2021 09:32:00",x1="12-10-2021 09:38:00",fillcolor="green", opacity=0.25, line_width=0)
fig4.update_yaxes(title_text='Fix Type', secondary_y=True)
fig4.update_layout(title_font=dict(size=22,color='black', family="Muli, sans-serif")

This gives me the plot shown below, everything is as expected, but i donโ€™t get the rectangle.
Any suggestions?

Did you try

from datetime import datetime

x0=datetime.fromisoformat("12-09-2021 09:32:00"),
x1=datetime.fromisoformat("12-10-2021 09:38:00"),
fillcolor="green", opacity=0.25, line_width=0)