It's entirely possible that the bullet was falling base-first when the plane hit it.
Actually, it's not. Look at the AV Herald photo, it shows that the bullet penetrated the skin of the LE flap in the usual fashion, tip first.
Also, bear in mind that the round in question looks like a FMJ military round; they will only deform if they strike a material that is harder than their jacket. The skin on the LE flap is probably 2090-T84 aluminium-lithium alloy or possibly 7475-T73 alloy; both are controlled toughness alloys with reasonably high strength that are fracture and fatigue crack resistant. However, they are not particularly resistant to penetration.
Given the minimal penetration and lack of deformation it does not appear to have been a particularly high energy impact. If I had to hazard a guess I'd say that the projectile was descending at the point of impact.