Thanks all for your replies.
I created the second style "Heading2bis" (based on the original Heading2),
and off course this worked. Probably next time I'll just do the manual
formatting (something I try to avoid as much as possible) as suggested in the
later replies.

Anyway, both are workarounds, and according to me, keeping the "page break
before" always cumulative is a badly implemented feature (it isn't a bug
though). It can't be that difficult to foresee this type of problems (it was
implemented for the "space before" and "space after", so somebody must have
thought about it).

Does anybody know whether this has improved in the new Word version that
will be released soon?

Thanks, Peter