If you are testing in a real world situation, there are a number of variables not typically present in a controlled environment, which probably account for the successful immediate casts:
1. Conditions for auto casting may not always be met (keep in mind the 20y range of auto casting).
2. If auto-casting conditions are met, you have about a 1 in 3 chance that you will hit the button while Eos is done casting and waiting for her cooldown to finish.
3. If the faerie is moving or starts moving to catch up to you after the cancellation, that could hinder auto-casting.
I conducted my testing in a controlled environment and created auto-casting conditions by changing equipment. Have you been doing that when you have tested out of combat?
For FC and FI under auto-casting conditions, the interrupt macro resulted in Eos re-starting Embrace, then using the ability 100% of the time.
Whispering Dawn, on the other hand, worked for me as before 100% of the time. Have you had any hiccups with WD in live situations?