Dieseling is when your engine suffers spark knock due to combustion chamber deposits glowing and causing combustion due to superheating (like a diesel engine).
You have a wiring issue with your fan. Not dieseling issue. Your fan motor is acting as a mini generator as the windings power down when you shut the key, as the fan is sharing same power as the coil, the fan is supplying some fly to the coil for a few spark events, there is already enough fuel in the inlet manifold to allow those combustion events.
Fix it up, or if your electrician is a dumbass, get him to install a rectifer diode on the ignition coil feed, I think khyber and cultus had it near the left shock tower. If its not then its very simple to add. Its very basic electric work for any auto electrician, make sure hes not “laktrishann”
In laktrishann lingo of Karachi a diode is called “dai wode”
A lot of these cars had the “dai wode” bypassed because thats the cheapest and most favorite way of repairs of anything in Pk, “nahi chalta, nikaaldo/bypass kardo - ye itumm ki zarooratt nahi hai, kaam to chalraha hai”
The above sentence is heard in car repair and even industrial systems repair in Pakistan.