A strange bug happened in my last 3 runs, Chani just disappeard from the game!
So I couldn't finish the game or cure the sick Fremen (who happened to be my main troups, of course).
I don't get how this bug happens. Does someone have a clue for this?

I found the cause of the bug eventually.
It happens if you meet Liet Kynes before leaving Gurney Hallek in a sietch to train Fremen.

Ughh this happened to me twice last few days. Except, I already had put gurney in sietch to train fremen well before meeting liet...

When the fremen get sick I put chani in the sietch and leave, and that’s it she disappears completely from the game

That's wierd. Are the Fremen still sick?
In the game's scenario, Chani gets kidnapped by the Harkonnen AFTER she heals all the Fremen. If the Fremen are cured (you get a telepathic message), go to the sietch and ask them where Chani is.

I can reproduce it. If you put Gurney in an empty sietch and send fremen to him the Duke doesn't die and after the Fremen get sick and you take Chani to cure them she just disappears. Fremen are still sick. May be coupled to another bug where you talk to a Fremen at Tuono-Calm to find the other village without finding the one in the fishes mouth.

Interesting. Thanks for explaining this way of reproducing the bug