Resist bug?

When I rolled over into cruel act 1 I noticed my cold resist was -5%. I killed the one early boss that nets you +10% cold resist, so I had 5% cold resist. So far, so good, but randomly and still in act 1 cruel my cold resist reverts to -5%. If I go back to the map where I killed that boss, my resist instantly goes back to 5%.

What is going on? I'm not switching any gear or switching acts or difficultly.

After finishing each act the elemental resistances get lowered by -10%.