I like Grace, but it has an annoying, occasional bug. When using the Mod Envelope, pressing Alt + turning the filter cutoff setting to make the Mod Env affect the filter, it occasionally resets back to zero. No explanation, it just sometimes does it when playing, sometimes when rendering, with no logic behind it. It's not the end of the world, you can always just turn it back to where you want, it's just mildy annoying.
Using Reaper 5.15, 64 bit.