Would that be a reason for G/A below 1000 ft?
No. Once you are below 1,000 feet then you are considered to have commenced the approach. Below 1,000 ft, monitor the aircraft performance - don't get distracted looking for something which may have failed quietly. Anything Airbus deems to affect the Landing Capability will be announced by "ECAM or a local caution".
The ISIS should be considered at the planning stage i.e. before you commence the approach. Treat its failure exactly like an RVR received after you have passed the approach ban point (1,000 ft).
QRH "Flight crews are not expected to check the equipment list before approach. When an ECAM or local caution occurs, the crew should use the list to confirm the landing capability."