The same issue for Smooth (Gaussian) which was reported in #109799 also affects the Butterworth filter.
When keys were not exactly on the frame, but offset even by minimal values the filter would introduce stepping.
Fix it by using `round()` instead of `(int)` when calculating the index to the filtered values array
Pull Request: https://projects.blender.org/blender/blender/pulls/110060
39cc72ea25