In the past, the engine has been made "smart enough" to not interrupt the restore process if some UDF library is missing. Instead, GBAK prints a warning and the process continues. But if some expression index references the missing UDF, the engine crashes while trying to call that UDF while creating the index.
We could throw an error and terminate the restore process in this case. Another alternative would be to restore such an index as inactive, with an appropriate message printed into stderr.