Hi,
I found the answer - 'solved by self', as they say.
It's an issue that is specific to FMCA Formbundle webdynpros - my visibility flags were apparently clashing with built-in behavior that switches fields on and off based on their prefix (eg 'C_DATE').
I solved it by putting the relevant fields inside a transparent container. I now control the visibility of the container instead.
Thanks for your fast and helpful responses!
cheers
Paul