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Putting this here for my future self…
I did some digging in the database, and found that the font definitions in FSE are stored in the posts table, with the URL escaped in the post_content field as follows:
{"src":"https:\/\/old.example.com\/wp-content\/uploads\/fonts\/S6u8w4BMUTPHh30wXiWrFCbw7A.woff2","fontWeight":"100","fontStyle":"normal","fontFamily":"Lato","preview":"https:\/\/s.w.org\/images\/fonts\/17.7\/previews\/lato\/lato-100-normal.svg"}The script used by my host to promote the site to “live” changes URL’s in the database but didn’t detect the escaped slashes in the URL.
Changing from old.example.com to new.example.com in these post_content fields resolved the issue.
But now I wonder if this issue would show up if I had changed the site URL using the WordPress Settings instead.