Off the top what would be helpful is to make available the actual character and rig source used prior to import as there are some basic rules that needs to be followed to ensure continuity of the source chain.
Rule one is the the animations must match the naming convention used on the rig as well as the key-frames loaded align with the rigs world translation else you will land up with the result in the above example. In this case the naming is right but the transform orientation is wrong (ie if your FK is left to right instead of up and down then the additive value is going to rotate the joint in that direction).
I’ve yet to see any info on the subject but knowing the joint orientation as to being able to use the animations out of the box is rather important if one is doing their rigging outside of the supplied tool chain.
The good news is since you did manage to get to this stage then all else that came before the problem is working properly and you can test the continuity of your current design by adding a few animations that matches the rig (full range of motion tests works great for this) and if it does work as expected I would recommend continuing to work with your own rig design and just replace the out of the box animation sets.