I doubt that it was a mistake unless they rendered the phaser beams as a separate render pass and composited it incorrectly.
I wish I had your optimism.
If I remember correctly the Defiant's
phaser shots from the lower sensor dome at the Avenger
in "In A Mirror, Darkly" would have penetrated the saucer itself, judging by the aft phaser beam angle (no Trekcore screencap available of the forward phasers firing).