As noted at T214974#4927834, https://gerrit.wikimedia.org/r/#/c/mediawiki/services/mobileapps/+/404510/ isn't actually a recent change and probably wasn't the proximate cause of the breakage.

For posterity, it looks like what happened here is that an edit to the rarely-edited enwiki beta main page triggered a RESTBase summary regeneration, which hadn't happened since before the mainpage summary type was introduced (in Jan 2018).

I don't remember clearly what the rationale was for adding the mainpage type, and it doesn't appear to be documented. I think it was intended as a kind of hint to clients about why the extracts provided would be (likely) empty. If that's not useful and there's no other strong product reason to flag main pages, it's probably best taken out.

I'm fine with leaving this as is.

Same here. Based on our guidelines for generic preview, I think this is expected behavior.

I dont quite follow this. For generic previews we show previews but main page links at time of writing is showing the error preview...

I understand declining on the basis that main page links are uncommon but not in this context...

In T215080#4939041, @Jdlrobson wrote:

Same here. Based on our guidelines for generic preview, I think this is expected behavior.

I dont quite follow this. For generic previews we show previews but main page links at time of writing is showing the error preview...

I understand declining on the basis that main page links are uncommon but not in this context...

My bad, I didn't read the copy, it's the generic preview "looks like there isn't a preview for this page" that should appear. I still feel like the two are close enough in meaning however, and this is quite a rare case.

I understand declining on the basis that main page links are uncommon

This was my thinking. Plus the current behavior doesn't seem particularly broken.