IF everything was how you described it and you didn't accidentally missed something, then you are right. There is no such thing as TAI speed correction for a normal F30 approach.
I would be curious to hear what kind of correction he wanted you to have and how you were supposed to calculate it.
The underspeed symbol might have appeared from some very simple reason, the A/T not keeping the speed right. What was your actual IAS at the time in relation to what you had set on the bug?