Just to make sure, you are referring to these?
Dodge Ram: XB LED - Fog Lights from Morimoto HID
The LED Projector fog lights? Or are you referring to an LED Bulb inside of a fog light housing?
I ran the X8 Projector fog lights on my 12 for several years, no resistors, no issues.
When i upgraded to my 18, i continued the plug and play approach, Just plugged them in. No issues, no resistors, no canbus cancelers, nothing. When i adjust settings for HID headlights the only fog light setting i changed was to disable the fog light drop out. I have had zero issues with the X8 Projector LED fogs
My Suggestion, with them haven worked for several years, is to check your grounds and your connectors. Sometimes the connectors might get loose, which can cause intermittent issues. Make sure your pins are fully seated and making proper contact. Especially since when you bench test the lights they work
The way the canbus system works, is at startup it will check the lights for any lights out. If it detects any issues, it will turn off power to that light, so the reason your factory light might not have worked at first is because it detected an issue, turned off power, then after a power cycle it checked again, and did not find an issue so it kept power on. I would lean towards the issue being more with the Morimoto light, or with the connector. Also, the morimoto staff is very knowledgeable and helpful. giving them a call might throw out an idea on something to check that none of us might think of that could solve the issue. If anyone else has ran into your issue, the manufacturer would be a fantastic place to start for assistance in troubleshooting.