It seems like null values in the middle of a series reset the stacking so that the next value starts at zero. I tried it upstream in a JS codepen and it is handled okay there, so I believe it is an issue with the python API or possibly with plotly.express.

Reprex modified from https://plotly.com/python/bar-charts/#bar-charts-with-text

import plotly.express as px
wide_df = px.data.medals_wide()
# set null value in middle 'silver' series
wide_df.iloc[0,2] = None
fig = px.bar(
 wide_df, 
 x="nation", 
 y=["gold", "silver", "bronze"], 
 title="Nulls Reset Stacking", 
 text_auto=True)
fig.show()

Output:

Based on input data:

Note that South Korea should be stacked to a total of 35 and not 24.