Example:

X-Report-Abuse:
 =?us-ascii?q?=3Chttps=3A=2F=2Fwww=2Emailitapp=2Ecom=2Freport=5Fabuse=2Ephp=3Fmid=3Dxxx-xxx-xxxx?=
 =?us-ascii?q?xxxxxxxxxxxxxxxxxxxx=3D=3D-xxx-xx-xx=3E?=

When this header is parsed and reencoded as bytes using policy.default, the result is this:

X-Report-Abuse: <https://www.mailitapp.com/report_abuse.php?mid=xxx-xxx-xxxx
xxxxxxxxxxxxxxxxxxxx==-xxx-xx-xx>

Since the header does not require its value to be an encoded word, it is written as plain text. This removes the FWS between the encoded words. However, it appears that it is still seen as a, now invalid, folding point. When an email with this header is parsed again, it causes a MissingHeaderBodySeparatorDefect