Altair themes break charts heights

For the sake of consistency, I would like to make sure that all my Altair charts use the same font as Streamlit – IBM Plex Sans.

As far as I can tell, the expected way to do that is via Altair themes:

So here is my code:

def streamlit_theme():
    font = "'IBM Plex Sans', sans-serif"
    return {
        "config": {
            "title": {"font": font},
            "axis": {"labelFont": font, "titleFont": font},
            "header": {"labelFont": font, "titleFont": font},
            "legend": {"labelFont": font, "titleFont": font},
alt.themes.register("streamlit_theme", streamlit_theme)

While the font does change correctly to IBM Plex Sans, the bad news is that my charts are now squashed. Look at their aspect ratios before and after:

Am I doing anything wrong? Any insights about how to fix this?