Misleading functions' name
Functions named with suffix Jacobian in BN254G2 library are in fact not working with Jacobian coordinates but with projective coordinates. The point-addition algorithm for implemented by the function _ECTwistAddJacobian is described by formula 3 of this paper↗, "Efficient Elliptic Curve Exponentiation Using Mixed Coordinates". The point-doubling algorithm implemented by function _ECTwistDoubleJacobian is the implementation of formula 4 of the same paper. The point-doubling algorithm was later optimized in hyperelliptic.org↗. The library may benefit from those improvements. In the same way, the function _fromJacobian transforms projective into affine coordinates.
The function _ECTwistMulByCofactorJacobian is an implementation of the algorithm from the paper "Faster Hashing to ↗" and is also not working on Jacobian coordinates.
The naming is misleading, and the lack of reference to the original algorithms may be error-prone for later developments.
Adding tests with test vectors coming from a source other than the library would improve the confidence in the implementation.
Session team implemented the suggested changes in with references to the implemented algorithms in the comments.