There have been CAPTCHA problems with Steam for at least a year. Search "steam captcha broken" and you'll see endless Reddit threads about it. Last week it took me two days to get it to work - the only solution was to solve the CAPTCHA from a mobile device in order to do a password reset. CAPTCHA just did not work on MacOS or Windows at all. After solving the CATPCHA on mobile and resetting the password, I was able to use it to login via Windows and MacOS.

To add to the problem, there are numerous "solutions" about flushing DNS caches, disabling IPv6, using the audio CAPTCHA, and other tricks that just did not work for me. I tried every "solution" in a dozen threads until the mobile suggestion finally worked.