I'm getting "Captcha wasn't filled" but not seeing any?

On trying to register accounts… only on some instances, not all, although I don’t remember seeing a captcha anywhere…

No one knows?

Can you provide links to some instances having this issue?

Ok. As displayed on the about page, this instance use the plugin mtcaptcha (by @rigelk ).
In Peertube 4.3.0 (last version, released past week), there is a new look & feel for the signup process. I think that the plugin is not compatible.
@Chocobozzz , do you think other captcha plugins can be broken too? I think there are multiple captcha plugins published by @rigelk (or other people) that works the same way.