Were both batteries charged externally? And what make and model are they?
Look at each battery's voltage right now. Unplug your mod, and swap the battery positions in your mod.
If cell 1 (old cell 2) still shows lower than cell 1, then we know something is wrong with that area. Either way you might want to clean all their contacts with Isopropyl alcohol (IPO), or even better, semi dismantle the mod and look for ejuice and wipe down the area around the bottom of the board (both sides), those 2 fatter connections and those 5 small ones. There might be white connector which you can pull apart off the board. Wipe down everything.
Looking at your screenshot, ideally/regularly they should be parallel. When you take a puff they should both dip/sag equally.
No, it's not dangerous, but it should be fixed. Because that % is based on the lowest cell, your mod will give you a weak or check battery much sooner than it should, and stop working.