This doesn't seem to be a recent regression, or related to the recent changes to mobile section editing. The same issue can be seen on desktop.

The problem is that VisualEditor renders all references in every references-list, while it should only render references up to this point. The initial rendering is from Parsoid, which does it correctly, so that's why it changes after you start typing.

(Related: T131563)

I was unaware of this functionality, although I think in practice this is never done because it is really confusing (and a horrible feature). If multiple ref lists are used on a page, they are each given their own unique group.

In theory we should support it, but it practice I can't see this ever being a good use of our time.

@Esanders provided the reason above of deprioritizing

In T228118#5337236, @Esanders wrote:

I was unaware of this functionality, although I think in practice this is never done because it is really confusing (and a horrible feature).

Indeed. I think the only sensible reason I've seen for it is if you're quoting a fragment of an article on a talk page, you want to display the relevant references directly underneath your section (and without references from other sections), without having to fiddle with the markup. (And this again is not a use case we really try to support in VE.)