Prior to SL Prepatch I had no issues with interrupts, but now only channeled spells are shown.
I figured out, that the GetTime() function returns the value in seconds and CastingInfo returns its values of start time and end time in microseconds.
I have fixed it for me, so the return value of the function Unit:CastPercentage is now:

return (GetTime() * 1000 - CastStartTime) / (CastEndTime - CastStartTime) * 100

Hello, thanks for the report, it's fixed in 2f73807