The insect line only receives the benefit of high night vision, which makes it very improbable for them to acquire infrared vision (prereq full night vision), which is marked as a characteristic mutation.
Mutations that are flagged in-category can pull prereqs. If it calls for IR and you’ve only got HNV, it should give FNV. Agreed that it’s not as certain as making FNV in-category, as it’s possible you could get canceled out for not-in-category before getting IR, but then part of the point of mutation is that results are NOT guaranteed.
Not a bug.