That's unexpected. Usually if the other party dealing damage is an NPC, the Player will always get full credit - at least in regular combat. Only if another Player is involved do we get this stupid "last hit wins" thing. When I was doing some AX combat last year (IIRC) I was pretty bad at it, relying on an Anaconda with AX Turrets. By careful (and tricky) positioning, I could angle my ship so the Turrets were able to hit the hearts and take them out. I was involved in an least one Interceptor Kill - I got the message it was dead, but it turns out I got no credit for the kill. No other players around as I was in a personal Private group at the time.
Not once taken down an Interceptor Solo, I'm really not that practised at AX combat, so I can imagine the frustration here after all that work. While NPC's deal some damage and occasionally land a hit on the hearts, it's the players (even with AX turrets) that deal the bulk of the damage.