I spent some time looking at the slider with FF Inspector too, and came to the same conclusions as ronpat.
On one of the containers it sets a fixed height, at the same time it sets max-height on the images.
At the smallest screen width I could go, I saw the fixed height of the container go down to 6px tall and the image go to max-height 4px.
A quick google search shows a lot of people having the same problem.
Divi Fullwidth Slider Height Adjustment
All the fixes or workarounds I've seen involve the use of
Others mention a complete replacement with another slider.