@Bryan, thanks for your help! The problem was that we had a plot element getting defined outside of the loop:
hline = Span(location=0, dimension=‘width’, line_color=‘dimgray’, line_width=1.4) # defines a horizontal line at y = 0
That was the object being shared by multiple documents, and throwing the error. Once we identified that and moved it under the loop, your solution worked just fine. Thanks for the assistance!