As it turns out, I figured it out after posting for help. Month_list was built for another workflow and on purpose did not include the current month. As soon as I had real data in the dataframe about the current month, the number of bins was off by one which caused some re-adjustments.
Still, it looks like the number of bins is adjusted in some ways by px.histogram internally since I often got half the bars I expected rather then n-1.