I have been having many issues since we moved past Octane version 2020.2. One of the more nasty ones is this motion blur issue. It happens unpredictably on certain frames. Thankfully it's consistent. I have tested this in the latest Octane and it is still a problem. The motion blur, for lack of a better phrase, "freaks out." The solution is to use the old deprecated Multi-pass motion blur. However this is not ideal, as it's slower and a no longer maintained feature. Also does not work with LWSN.
Load scene. Go to frame 282. F9. Now render frames 281 and 283. Or, render it all if you like. You will see the "whacky" frame. Again, this can be fixed using the older Multi-pass motion blur.