I’m using Publii 0.43.1 with Simple 2.8.2.0 (though this issue has been present for some versions now).
If I put a featured image in a post and the site settings are responsive (and/or webp), then the hero image is correctly displayed responsively and converted to webp format, but the individual post featured images are not.
I believe this issue is visible in the live demo of the Simple theme: https://demo.getpublii.eu/themes/simple/mountains-simple-tips/