Do you happen to be on a release prior to V5R3? In V5R3 the limit became 4GB as shown here.
We are on V5R4
I am not familiar with the ASC product, but when an open to the IFS is done the oflag parameter supports the setting of O_LARGEFILE. Per the open flag API documentation:
Open a large file. The descriptor returned can be used with the other APIs to operate on files larger than 2GB (GB = 1073741824) minus 1 byte. The file systems that do not support large files will just ignore the O_LARGEFILE open flag if it is set. The O_LARGEFILE flag is ignored by the file systems when open() is used to open a directory.
So ASC is evidently not setting the flag to enable support for greater than 2GB (basically treating file offsets as signed rather than unsigned integers).
I would suggest contacting ASC.
I will check with ASC