furnace in igloo melts ice block on side

i was using an igloo as a temporary home, and used its furnace to smelt iron, but when i did, the ice block next to it in the side of the igloo began to melt, which was odd because previously, i thought they were supposed to be packed ice rather than normal ice, and the melting caused water to go everywhere outside of the igloo.

What I expected to happen was...:
the furnace to not damage the igloo

What actually happened was...:
the furnace melted the ice block closest to it when used
Steps to Reproduce:
1. find an igloo
2. use the furnace inside
3. wait for the ice block to melt