The bear effect

Not a kata suggestion. A lot of katas have very biased upvotes, since the top-voted solutions are always at the top, they get upvoted constantly, but that does not mean they are "best practices".

I am suprised nobody used a set to solve the problem. All the top one liner solutions have a complexity of O(m * n), which is inefficent for large inputs

Not a kata suggestion. See https://docs.codewars.com/training/troubleshooting#post-discourse.

Not a kata issue, the code you think is the right one does a different thing to what the kata asks:

It should remove all values from list a, which are present in list b keeping their order.

If a is empty, your code returns b.

I do also believe that this is too easy for a 6 kyu, being more like a 7 kyu.

you are asked for this: https://en.wikipedia.org/wiki/Complement_(set_theory)#Relative_complement

you are doing this: https://en.wikipedia.org/wiki/Symmetric_difference

i am close to giving up at this point, filtered the 2 arrays to get the unique values for both and the returned the concatenated unique values as an array, i even passed the sample arrays and got the exact same answers, but it's still not accepting my answer

Not sure what your question is, but seems like you've solved it, so closing...

Missing using System.Linq perhaps...