Why a simple zero-pad introduces errors in my simulink transmission chain?!

I'm working on the DVB-S2 transmission chain, taking advantage of its implementation already available in the Simulink Communications Systems Toolbox demos (commdvbs2). In particular, I'd like to add filter shaping after modulation. However this doesn't work, as errors are introduced.

I found that a first (hopefully the only) problem is due to the the simple zero padding operation or, better, the subsequent truncation in reception to discard the padding. Hereby I attach a very trivial schematic in which we can notice the arising of an error rate by simply applying zero-padding and filter shaping (if I remove one of the two operations, it works).

Any help about this? Thanks a lot Luca