So the rules doesn't work for Timeless, it couldn't properly hide the first heading of Timeless; when tagline is manually enabled through CSS it collides with the first heading.
Topic on Extension talk:SocialProfile
First off, the mandatory disclaimer: only the latest LTS release (1.39) is truly tested and supported, support for any and all other versions is coincidental at best. :-)
Now...
This looks like it might've been caused by commit e733d2624a03eb05c19240e6302939a2c288dbfb on the Timeless repository, which for sure overrides the .firstHeading rule from SocialProfile's /extensions/SocialProfile/UserProfile/resources/css/UserProfile.css. Maybe something like .skin-timeless h1.firstHeading, in addition to the existing .firstHeading rule in that file, would do the trick? Let me know and I'll be happy to amend the file accordingly. (Although I'm not fond of skin-specific hacks, I'm hoping that there won't be too many such similar cases; but as of MW 1.39 pretty much all skins, even Timeless, behave as expected, so...hard to tell, really.)
@Jack Phoenix Welp, as long as you could manage to manage the whole project structure more stable and easy to maintain, a fix on your side would be preferred. For now I'm using a pretty bad way to detect whether the user profile ResourceLoader module is loaded and then load a fix style accordingly if yes, and I must say that I'm not satisfied with this https://lakeus.xyz/wiki/MediaWiki:Gadget-userprofile-fix.js. I must say though, why not just add an !important to display: none ? There are already some of these for some unknown reason to me anyway.