I had Ancient bark and flint go through enemies as well, its likely more enemy specific. Later enemies are faster and smaller. Never had it happen to boar or neek too.

But yeah its a problem, can confirm. Spear randomly goes completely through a mob, even at like 2-3m ranges too, on a single player world.

Thou notably, the later spears are far from usleless, the projectile launch point, as with bows and crossbows, is FAR in front of the player, so when targeting a close mob thats just below, you have to take BIG correction, having crosshair at the mob is not enough to make the projectile actually go through where you are aiming. You need to look further down.

Be sure and complete a real bug report in the appropriate places, probably the Valheim discord and/or the steam bug discussion.