I exchanged at first 200, then later 100 Cortex Fragments at Rebus.
At first, I had 298, the shown number did not update.
Then I noticed 200 fragments were gone nevertheless. I did NOT gain any favor.
After adding some more favors from another character I tried to exchange 100, I got the "exchange 100 fragments" prompt, the shown number did not update, 100 were taken, ZERO favors were added.
300 Cortex Fragments down the drain.