That doesn't explain me why, if I enter the (timed out) Captcha and press Preview I'll get the "wrong captcha" error, right? ;-)
I shouldn't get any error as, on Preview, Captcha shoudn't be verified, IMHO.
I never tried to avoid writing the captcha too, even before a Preview so, I bet, captcha is verified, even if you press Preview, only if you change it.
My suggestion to the IT Team then becomes "just visual": to move the Preview button over the Captcha section, which should mean "you don't need (and shouldn't) enter captcha to Preview", and to place the Submit button at captcha section's right side, this way:
(Preview)
Captcha [image] [text field] (Submit)
If they would like to avoid captcha check even when, before pressing Preview, one changed it, it's up to them.
P.S.
Thank you, Rebeka, to pass the suggestion along to IT Team. :-)