It comes from the DiscussionTools extension CSS:
https://github.com/wikimedia/mediawiki-extensions-DiscussionTools/blob/0b877fba05b8a2d984e84100695196d797ec1bc3/modules/dt.init.less#L358-L360

Judging by T313636: Add space between talk page namespace and page title, it looks like it might be intentional.

Thanks, based on your linked task this might be intentional for talk page namespaces. But the issue remains that NEWSECTIONLINK creates the same space in other namespaces while pages without NEWSECTIONLINK appear differently. E.g.

https://en.wikipedia.org/wiki/Wikipedia:Reliable_sources/Noticeboard?safemode=1

vs.

https://en.wikipedia.org/wiki/Wikipedia:Reliable_sources?safemode=1

And why is it only displayed that way to logged-in users?

In T396928#10915354, @Johannnes89 wrote:

And why is it only displayed that way to logged-in users?

Ok this is explained in T338151 (although I don't understand why the DiscussionTools usability improvements are still not rolled out to logged-out desktop users two years later),

But the confusing inconsistency between pages in non talk page namespaces using NEWSECTIONLINK and those not using it remains. Doesn't seem like the space helps people identifying pages as talk pages.

In T396928#10915354, @Johannnes89 wrote:

And why is it only displayed that way to logged-in users?

Because the feature adding the spacing is currently a beta feature (part of the Discussion tools beta feature, which I assume you have enabled) on enwiki, enabling it by default is tracked in T379264: Phase 5: Offer Usability Improvements as default-on feature at remaining large wikis.