My CSS knowledge isn’t good enough to say why Safari is “doing different”. I can only comment that the current (Monterey) version sees Safari in one of its weird periods. As a dedicated Safari user I find that the app sometimes plays nice with almost all websites, and sometimes it doesn’t. We seem to be in one of the latter periods at the moment. Good luck.
I haven’t found much writing on it, but playing around with Safari, it seems to do some odd calculations with the color of a disabled input’s text based off its background color. Explicitly setting the -webkit-text-fill-color property seems to fix it. This is an issue across all of Omeka S’s disabled inputs, so I’ll put a fix into the core.