Kerning classes and font generation

Hello, and happy holidays to everyone,

I have kind of a noob question, but hey, you gotta learn, right?

So I'm using Fontlab 5. I open the "New Metrics Window", I type my pairs (let's say "LT"), adjust the kerning manually. Then I create my kerning classes with the diacritics. Then I generate the "kern" feature in the OpenType panel. Then I compile it. Whenever I generate the font file (File > Generate Font), my kerning is completely f*cked up. When I open the .ttf file, I can see some classes have disappeared (only one among the two I've created remains in this particular example) and some kerning pairs have disappeared too. The "kern" feature isn't the same anymore and its whole code has changed.

Now I think I could expand kerning to create a flat kerning table to avoid that, but I was wondering why is this happening. Am I doing something wrong, or is it some sort of settings to adjust in order to keep the features/classes as they are when the font file is generated? I'm wondering because since I saw this issue, I saw it in every font I have created in the past (pairs disappearing, classes being renamed too, "kern" feature being edited).

Thank you for taking the time to read my question, and thank you for your help!