Pawns will not attack certain types of bugs (Megascarabs and possibly Megaspiders) even when bugs are attacking, drafted pawns will stand still and allow themselves to be attacked.
I've seen this issue in at least 1.3.3069 but haven't encountered insects in my current colony for a while. Pawns don't always consider insects (possibly just spelopedes) hostile, and so their aggression response (flee, attack) sometimes doesn't trigger. Drafted pawns sometimes allow insects to pass to attack another colonist without fighting back, and insects sometimes pass their own engaged in combat in order to reach and attack another colonist in the back. Unfortunately I haven't looked into reproducing this issue yet.
Just an update to this, in both of the last 2 bug attacks paws did attack as expected.