It turns out that KDE's "Reject Cross-Domain Cookies" feature of the KDE Libraries has been broken since 2002 and was only fixed yesterday. Thiago Macieira dived into the problem yesterday to see why all of his web cookies would be forgotten after a kded restart. The problem originates within KDE's KCookieJar within the HTTP KIOSlave of kdelibs.
Per Revision 794b14b8 of the KDE Libraries from yesterday:
Initialise the mCrossDomain member variable in the cookiesTip of the hat to Phoronix reader "markg85" for pointing out this interesting, long-standing bug of KDE.
For several months now, all my cookies would be forgotten after a kded restart. After debugging the problem, turns out that mCrossDomain was of value 127, which makes no sense for a boolean.
This variable has been present since 2002, which means that the "reject cross domain cookies" feature has been broken for 10 years and 8 months.