With a few of my new websites I have noticed that google is not showing what is truly indexed when using the site: operator. I will show you an example.
With this website google shows only 17 pages indexed when using the site: operator. On mobile I only have 10 results indexed using the site: operator. But on desktop (only desktop)
at the top of the search result page, Google says there are 76 results indexed. I can confirm this because I track my keyword rankings. I am ranking for all of my keywords I have targeted (or some variation) on both mobile and desktop.
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So check to make sure this isn't what is happening with your website before you jump to any conclusions.