Yup, once over the head is not enough for a valid signal.
Perhaps you're right, but the covering official did not flag an invalid signal by the receiver. He flagged KCI by the kicking team.
I wondered why this would be KCI and found AR 6-4-1-VII:
B10 signals for a fair catch, muffs the ball and then catches it. RULING:
If B10 has an opportunity to catch the kick after the muff,
he must be
given an unimpeded opportunity to complete the catch. If B10 catches
the muffed kick, the ball is dead where he first touched it.
I take it that there would have been no foul on A in this play had B been flagged for an invalid signal.
For NFHS this would not have been KCI. The opportunity to catch the kick ends with the muff.