As a Werewolf who is immune to non-magic physical damage, can Swarm of Rot Grubs get past my skin

I’m a part of a homebrew setting 5e game where the real homebrew, outside of lore, is only custom monsters that are rarely made by the DM as he prefers to use normal dnd monsters when he can. In this latest game, he used Swarms of Rot Grubs and one managed to hit me and a turn later dig in.

However, I questioned how the Grubs got through my skin in the first place as normal swords and spears failed to pierce my hide in the past.

DM ruled it as they do, so I will accept it as such….also helps we’re fairly mid-game, so we have someone with lesser restoration.

But I want to know how the internet thinks about this, since the Grubs have no way of hurting me, can they still somehow bypass that Immunity to feast on my heart?

DM allows content up to Tasha’s. From what I understand it’s the basic version of DnD 5e. Apologies if that doesn’t clarify things better.