When you create two meshes that have overlapping faces and set alpha threshold and alpha below the threshold on one, it’s not rendered properly with HDRP. I submitted a bug report about this with a small reproductible: ID: 1323760
I tested this on 2021.1.1f1 and 2021.1.0f1.
I already created a thread about this but in the wrong section, it appears I cannot move it.