Your 1st mistake: Sending just
ONE Interceptor
Sorry for the rude comment, but even on easier difficulties I usually don't engage harvesters at all unless there on MY home turf or without multiple interceptors.
Now as for the "Glitch":
1) Maybe it was just a rare glitch, I haven't seen it yet.
2) Maybe the interceptor didn't have enough ammo, which is impossible at the beginning of the game.
3) Cannons have the
Shortest Range in battle, you got shot to hell before you could get close enough.
All of my interceptors have one missile launcher and at least one cannon.
Hope I helped a little.