I walk you through the steps what I have done right now.
Like in your link described, I created a new material named Custom2 on the printer
Exported it to SD
This is what it looked like in material.txt: [material] name=CUSTOM2 temperature=210 temperature_210 temperature_195 temperature_230 temperature_240 temperature_240 bed_temperature=60 fan_speed=100 flow=100 diameter=2.85
Then I edited the material like this: [material] name=NYLON temperature=255 temperature_255 temperature_255 temperature_255 temperature_255 temperature_255 bed_temperature=45 fan_speed=0 flow=100 diameter=2.85
Imported the material from SD. I have to do the import twice!, the changed material name appears only after the second import in the list!
Then I exported the materials to SD again
And to my surprise the material data is now a mix of my settings and some of the temperature settings that were originally stored in material CUSTOM2: [material] name=NYLON temperature=255 temperature_210 temperature_195 temperature_230 temperature_240 temperature_240 bed_temperature=45 fan_speed=0 flow=100 diameter=2.85
So it seems that the import function doesn't properly write all the parameters into system memory. Even changing the temperature setting of NYLON then on the printer directly didn't make them persistent.
After furiously switching the SD card between printer and laptop for a while now this seems to be the only way to make it work at the moment:
Select Customize on the printer, adjust settings and save as new preset
After this step, the nozzle temperatures for this material cannot be changed persistently, neither on the printer, nor with material.txt
The name and the rest of the settings can be changed on both the printer or the materials.txt
On a side note, the Export to SD function crashed the printer firmware four times over the course of this test.