When there is a string variable in the ToolValidator class of a script tool created from a project template, the variable is replaced by the following Esri variable, g_ESRI_variable_1 = os.path.join(arcpy.env.packageWorkspace,), which causes the path variables to become incorrect and the tool to break.
The reported issue, as described, cannot be reproduced in the currently released build. This defect is being closed as non-reproducible as the data needed to reproduce the issue are no longer available.