From the 2 July 2018 FAQ at www.W5SWL.com
You are probably not the only one that this has happened to, myself included. Melting the connector insulator and then blaming it on a good connector is a NO NO. And never admit it in front of friends! And it doesn't nessessarily mean the connector was a cheap China import :)
One of the main reasons we see in hands on experience when the insulator on a connector melts (other than a fake China connector) is that you are using too low of heat and applying it for too long. Yes, the heat is too low and to get the pin and wire hot and the solder to flow you have to hold on the connector for too long. You transfer a lot of heat this way to everything. Even your fingers! And both your fingers and connector insulation can melt.
It is far better to use high heat so that you can get on and off the solder connector FAST.
Here are some melting temperatures of some common connector insulators, I know my bench tools will far exceed these temps.
Delrin : 347 F
Teflon : 620 F
While you are thinking about it, what temperature does your solder melt? You don't really want to do a connector install with a high temp solder!
Using a gun to solder is not the best way to do connector work. The gun has no thermal mass, and as such it loses temperature really fast when applied to the connector pin and solder. If you are going to use a gun, get it HOT and apply solder to it first to get it melted, then touch the connector pin. I personally use a gun and trust me, I have botched many connectors with that gun.
The really good workable way is to use irons with a good temperature control and with sufficient mass to maintain their temperature when touching the solder and pin. Remember your iron has to keep itself hot, heat the solder, heat the pin and heat the center wire. Sometimes it takes two irons. Google soldering PL-259s and you will find a wealth of information.
Solder needs to be within the melting range of what you are doing temperature wise. It won't work if your solder melts at 450 degrees and your connector insulator melts at 350 degrees. One type solder doesn't fit all needs unfortunately.
Did you know, on a PL-259 you only fill the center pin tip with enough solder to close the tip area? I didn't know that either for many years and made it harder on myself getting connectors installed without melting insulators. Just solder enough to solder the end of the center wire and tip together at the end of the connector tip. If you try to wick the solder up into the whole pin, you are asking to melt the connector insulation.