Thanks @_vincent. I just gave it a try on browserstack.com as there’s no Safari 12.x currently around us.
Results for Safari 11.1 and 12.0
https://www.browserstack.com/screenshots/9438a3becc3af763386c60bd955bf1c91bd78ae2
Thumbnails
OS X High Sierra: Safari 11.1
OS X Mojave: Safari 12
This looks reasonably good to me. Do you have an idea what could have gone wrong? Do you have more details about your devices or setup, if this might actually matter resolving this issue? I feel a bit silly now.