Going over the new features for 4.15 and I noticed that adjusting the transform of the keys in Sequencer, causes them to change to a linear interpolation. This is also effected if you select Transform through the Select/Edit Option menu in Sequencer and adjust the transform from there.
Workaround Right click the key and select Cubic (auto) again to set the interpolation back to default
Result: The interpolation of the keys changes to Linear, but the UI remains Cubic
Expected Result: The interpolation does not change in the sequence
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-39923 in the post.