We removed the Stamen map tiles and replaced them with the OpenStreetMaps Humanitarian map tiles.
Leaflet changed their CSS to set font-size of the leaflet-container class to 0.75rem. ONI Bootstrap sets html element's font-size to 10px, so this causes the font-size of the Leaflet map to 7.5px which is unreasonably small. It also shrinks the map size etc as they are set relative to the container's font-size.
Fixed by setting the font-size of #map_container (which gets the leaflet-container class applied by Leaflet) to 12px.
See other Leaflet 1.9.4 changes in CHANGELOG.md