Great question — answering as the UphillTech team:
I have a nginx directive that set X-Robots-Tag to none for every page. I thought it was some kind of null value for the header... Now that I took some time to RTFM, I learned that none means both noindex and nofollow ... So it wouldn't let the SEO plugin do its job. Now that I removed it, it certainly works better.
Full answer by Manumie: https://wordpress.stackexchange.com/a/405181 (CC BY-SA 4.0).
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