The sitemap is valid but is not indexed and Google is not taking it.
As noted before - the only apparent difference is the xmlns:image= which shouldn't be an issue.
Also inspecting raw headers you'll see that on all accepted sitemaps then you get
but on pages_1.xml you get
Which is probably a code name for things which are bust or have been bust. My guess is that possibly there was an error before here which means Google is now ignoring these. From what I gather this can happen.
Anyway - If you try to submit /sitemap_pages.xml and not /sitemap_pages_1.xml - you're in business.
I'm guessing if you've got a tonne of pages then you'll be hit by a pagination limit but for my purposes today it's fine.
sidenote: it wouldn't be much work to build a new pages xml sitemap - if you need help give me a shout.
This has been an ongoing issue since Feb/19. I am starting to doubt the Shopify is even watching this thread. If they are they don't seem to think it is an issue. I would recommend you call support and report the issue. The more people that they get reports from the more likely they'll do something about it.
I have been wondering the same thing. I noticed that adding pages into the Search Console it takes them and says it will index, but all my pages that were supposedly in the sitemap that search console says it indexed are just no there. So if someone does know how to build a new sitemap I think it would be worthwile. I was thinking of changing to a Wordpress site as the cost is lower and probably does not have this issue. Certainly cannot stay in business if the largest search engine does not have your pages
I just noticed this myself, same issue here. I did notice that Google indexed my Pages anyways, but my attention to the issue was raised when I was looking at the "Coverage" section in the Search Console, and saw all my Pages in the "indexed, not submitted" section.
Thank you @Liquify for the workaround tip.