According to the documentation of dd, "iflag=fullblock" is required only when dd is used with the "count=" option.

Otherwise, i.e. when dd has to read the entire input file because there is no "count=" option, "iflag=fullblock" does not have any documented effect.

From "info dd":

"If short reads occur, as could be the case when reading from a pipe for example, ‘iflag=fullblock’ ensures that ‘count=’ counts complete input blocks rather than input read operations."