If there was a heart on the window that you removed when you moved in it may be that some residue of what was used to attach it was left behind or that space can't collect moisture.

Frost is formed by warm moist air collecting on a cold surface. The moisture in the air then freezes creating frost

The slower the frost forms, the more intricate the crystal structure becomes.

As the very still layer of air over the glass is cooled by the glass, water vapour desublimates out of the 100% humidity air directly onto the crystal formation (rather than randomly onto the glass). It is a slow process because the air is very still and it takes time for fresh 'moist' air to migrate to the glass and continue the process.

The base for the whole frost structure usually originates from dirt or some other residual on the glass (fingerprints, ahdesives, etc.) and it grows from there.