This is driven by enhanced tracking prevention. If you turn that off for the respective site, then it goes away.
Pretty sure I try disabling protections in such situations but maybe not. I returned to the last site that did it to me to try this out (on a different machine) and it didn't captcha me at all with protections on! Ugh.
This is driven by enhanced tracking prevention. If you turn that off for the respective site, then it goes away.