Hi! I just checked this issue, and I can't seem to replicate it locally (on a 15.9 SNAPSHOT distribution – I doubt a fix would have happened in the meantime). I triggered the captcha to appear on the login form, and the answer field is properly keyboard accessible.

I checked with Captcha types image and text. I also checked the 'register' form just in case but its Captcha is keyboard accessible too.

The answer field is an input with seemingly standard attributes that wouldn't break accessibility.

Could you give more detail on the place and conditions you found this Captcha with?

Thanks

Lucas