Coincidentally it was time for an upgrade, so I bought a new iMac. Fresh out of the box, I installed Chrome, Avast, went to load Patreon and it asked for CAPTCHA just like it did on my other desktop.
It doesn’t do this on my mobile or older desktop with outdated Chrome, only on two iMacs with updated Chrome.
Went to Preferences > Core Shield > Web Shield and put an exception for http/https at patreon.com and I have refreshed my open Patreon tab a few times.
It seems to have resolved the issue!
I just hope no one finds a loophole through Patreon now!