That is a bug with either uosc or the thumbfast script. You should open an issue there.
I was expecting that it has something to do with evafast, but sure I will try to see where the problem come from.
Edit: nevermind, it's already an opened issue in evafast
When I set a custom speed like X1.46 for example the I fast-forward with the arrow keys the video speed resets to the default X1. OS: Windows 11 23H2