I'm pretty sure that's been fixed, can you try 21.4?
(demo.t.o says "Version 21.3vcs is no longer supported. A minor upgrade to 21.4 is strongly recommended.")
when I try the contact form with a 21.4 locally, there is still the empty captcha value message.
A workaround at the moment is to use google recaptcha, which I consider as an ethically dirty workaround. (Seriously, those guys ask my phone number tu use an email )
Do you know when/if demo.tiki.org will switch from 21.3 to 21.4?