Google Calendar API sends recaptchas for your robot to solve (?)

For 1 hour since 00:45 GMT+10 I got a weird HTML response from Google Cloud Calendar API (which was supposed to only return JSON strings) saying "Our systems have detected unusual traffic from your computer network. This page checks to see if it's really you sending the requests, and not a robot".

Well... it's an API so it will always be a robot.

There was no unusual traffic, probably a bug on their side. It's weird that they would return a captcha HTML via thei API instead of an HTTP 429 Too Many Requests response status code.

It makes me wondering, is this common practice nowadays? Or is it just shit engineering from their side?