I honestly don't think Shopify can actually do anything here as such else I would have escalated this. So let's not bury the answer in moaning about it (as most people will skip to the end looking for a solution and not find the actual solution).
The sitemaps are all technically valid. Google is simply ignoring the page one. So the issue is more with Google. My guess is that for a time in the past these page sitemaps had an error and so Google stopped fetching them all. Either that or the big Google machine has decided that content under this sitemap is routinely not of value on Shopify sites.
Either way - just use my previous solution to solve it. End of.
Generally considered best to not continue forum threads if there's a solution as it kinda buries it .
Same problem going on here. I submitted the "pages_sitemap" without the number "1" like: www.domain.com/sitemap_pages.xml and all seems to work fine.
@AquaLiv - If your objective is to successfully submit your Shopify Page URLs to Google for indexing, our friend @Liquify has THE SOLUTION to your problem >>> remove the "-1" from the Pages sitemap and submit it separately, like this...
From an SEO perspective, whether Shopify / Google ever fix the issue preventing the -1 version from being submitted is irrelevant. If you want Google to crawl and index your pages so they have the ability to rank, follow the advice above and move on to more important SEO tasks :)
And if you or anyone else is looking for more info about this issue and how it relates to your SEO, check out my reply in a similar Shopify Community thread.