ACCBIAS is used for System Managed Buffering - SMB. Firstly the VSAM dataset must be SMS managed and must be defined as an Extended Format dataset in the appropriate Data Class.
ACCBIAS = SYSTEM really leaves it up to VSAM to decide on Buffering with no undertanding if the VSAM processing of the dataset needs more Index Buffers or Data Buffers.
This is not the best use of SMB. If you know that the dataset is being accessed more in Sequential mode then more Data Buffers are needed. So specify ACCBIAS=SO
If in Randon or Direct mode then more Index buffers. So specify ACCBIAS=DO.
In any case any Repro that has VSAM as an output dataset should be specifed as ACCBIAS=CO - CREATE OPTIMIZED. This will work as long as the Cluster is defined with SPEED. If the Cluster is defined with RECOVERY, then VSAM internally will use CR - CREATE RECOVERY OPTIMIZED.
How do you know what SMB option VSAM used then add this JCL DD keyword MSG=SMBBIAS and VSAM will tell you on the Joblog.