It's because that Garageband runs in a sandbox,
Please copy the contents of the PositiveGrid/BIAS_Amp2 folder in Documents to the location below to solve this issue:
/Users/*username*/Library/Containers/com.apple.garageband10/Data/Documents/PositiveGrid/BIAS_Amp2
*Please click on the 'Go' menu in Finder while holding down [Option/Alt] to access the'username/Library' folder.*