Recently I came across a couple portals that had all but one or two resonators destroyed on them and was surprised whoever started taking over the portal didn't finish the job. Then I tried to destroy the last two resonators which had very low energy, and could barely make a dent in them. My level 3 xmps were doing 1% - 2% damage to level 1 resonators and my level 1 xmps had no effect.
This happened to me on 3 different portals, where the last resonator or two was incredibly difficult to destroy.
Some people in chat seemed to think this was a bug, but I'm wondering if maybe the gameplay is just changing? Maybe there's a special new shield type I haven't seen before?