I've been a happy user of the /2.1/similar StackExchange API for a while, but at some point within the last 24h or so it started returning empty results (but otherwise reporting everything as good).

I am seeing the same behavior from the doc examples; https://api.stackexchange.com/docs/similar#order=desc&sort=activity&title=help%20with%20stuff&filter=default&site=stackoverflow&run=true

It does not matter which site I search or which search terms I use. And yes; I triple checked that I am not busting the rate limit or otherwise getting errors reported :-)