In OPC UA terminology, the quality (status) of a variable is used to communicate whether the OPC UA server was able to read or write the value to / from the source.
So in this case the bad variables are configured somehow incorrectly and the KEPServerEX was unable to read or write them from the PLC. This could be due to wrong tag name, incorrect data type, the PLC refusing access etc. The driver documentation from Kepware may have for more useful info on how to troubleshoot the issue.
You are using this driver, right? See the documents available from here:
https://www.kepware.com/en-us/products/kepserverex/drivers/siemens-tcp-ip-ethernet/