In most 'current spec' Airbii FBW a/c that I'm aware of, on selection of TOGA the FDs come back on and ATHR arms.
Now earlier software specs did have some subtle but important changes so I think it's important to find out just what spec a/c (or sim) you're talking about .
We had a situation in our fleet where an a/c that we had acquired from a previous operator (iso direct from AI) had a different response to TOGA application and sounds similar to what IO has described! It was an exciting ride for the crew (admittedly many moons ago now) when all the thrust came off!
With current spec a/c this has been resolved.