Reproduction steps:
1. Open attached project "FbxExport.zip"
2. In Project window, open Assets/Test.cs
3. In script editor, observe lines 13 and 17

Expected result: No errors, FbxPropertyString.Create() creates type of FbxPropertyString, FbxProperty.Set() can use a 'string' parameter
Actual result: Error, FbxPropertyString.Create() creates type of FbxProperty, FbxProperty.Set() can't use a 'string' parameter

Reproducible with: 3.2.1-preview.2 (2018.4.30f1), 4.0.0-pre.1 (2019.4.18f1, 2020.2.2f1, 2021.1.0b2)

Notes:
1. Issue also reproduces with other parameter types, such as 'bool'.
2. As a workaround, it's possible to add a custom script to the package's base class 'FbxProperty':

public FbxPropertyString Set(string pValue)
{
FbxPropertyString ret = new FbxPropertyString(NativeMethods.FbxPropertyString_Set(swigCPtr, pValue), false);
if (NativeMethods.SWIGPendingException.Pending) throw NativeMethods.SWIGPendingException.Retrieve();
return ret;
}