The site was reporting many JS/CSS files as 404 (this was with the MMR plugin, but I’m pretty sure that was coincidental).
404: wp-content/mmr/0123abcd-1234567890.jsSwitching to a twenty* theme with 0 plugins enabled didn’t result in any change, so it seemed like a caching issue.
There was a normal install of WP Super Cache, as well as a “drop-in” install of advanced-cache.php. Deactivating/disabling these had no effect.
It turned out to be a cache at the webhost (SiteGround in this case). I disabled the ‘nginx Direct Delivery’, and flushed the Dynamic Cache, and that resolved the issue (both under Site Tools > Speed > Caching).
After reactivating all plugins and WP caches, the issue re-appeared, but another cycle of flushing the cache at SiteGround SiteTools seemed to have resolved it again, and that fix has persisted for 10+ minutes.
