For example, after that "normalization", quoted printable encoded
headers (as described at rfc 2047) longer than 76 characters are
splitted in two different ill-formed headers because the soft LF line
break becomes a "hard" CRLF one. This is clearly wrong.

rfc 2060 specifically allows CR and LF inside literals:

"""
A literal is a sequence of zero or more octets (including CR and LF),
prefix-quoted with an octet count in the form of an open brace ("{"),
the number of octets, close brace ("}"), and CRLF.
"""