Hello all,
@BFD Calendar - this did actually the trick. If no user is logged in the textblocks render once as expected. Thanks for the hint!
I also figured out, that this behaviour depends on frontend editing, which is enabled for this particular field in my case. Once this is disabled, it also works fine even when logged on to processwire's backend.
For my scenario I m fine, however, I think it is a small glitch and could be enhanced... any opinions if I should file a bug report for this and how and where?
Many thanks!