This seems to be a caching issue, caused by the Jetpack uptime monitor. What happens is, when a request for the site’s homepage comes from the Jetpack monitor server, the site returns that text instead of the actual site content.
However, if that page gets cached upstream somewhere (Sucuri, Cloudflare, Varnish, wherever), it can end up being served to all visitors instead of the actual homepage.
This can probably be fixed very easily for everyone forever on Jetpack’s side, by having the monitor server add a query string to the request url like https://example.com/?jetpack=1 so it wouldnt be cached for visitors not using that same query.
Turning off monitoring in Jetpack will also fix it on a per-site basis, but then you don’t have site monitoring sooooo..
