The key was learning that the called program was from the 1970s, thereby explaining that ENVIRONMENT N was truly fine as it is not LE COBOL. The resolution was therefore to check the AMODE parameter.
Confirming it was set to CALL(AMODE31), changing AMODE31 to N for the option file via the JOB06OP2 JCL in the CBAAJCL library or by adding CALL AMODE24 to the PARM statement of this particular program resolved the problem.
Documentation link for the AMODE31/AMODE24 parameter is documented via this link:
Had there not been a successful override on the PARM statement, the solution would have included creating a second option file with AMODE31 N and added an //EZOPTBL DD statement to the JCL of the program and also ensured via JOB06OP1 that the correct option file is still linked into the EZTINI member of the CBAALOAD library.