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 .
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