[Q]

I probably found the problem: The combat to hit defense/offense ability uses a group ID (amount 2), which decides if two values of the same ability stack or just the higher value is used. If the ship already has a combat sensor/ecm then the bonus gained by the ship/fleet experience will have no effect.
The solution: give the ship experience bonus the value 6 as amount 2 for the bonus and for the fleet bonus the value 7. Then they stack with each other and master computer/ecm/combat sensor/stealth and scattering armor.

At least that is what I found so far. Any confirmation would however be appreciated.