For the sake of completeness, here is how I solved my problem.
It turned out to be a memory issue which was resolved by using the SNAP performance optimisation tool (I was directed there by this interesting forum post : GPT batch processing abnormality). It is located in snap -> Preferences -> Performance. Running «compute» on the System parameters adjusted the java heap limits (from the default -Xmx5120m -Xms256m to -Xmx1468m -Xms1468m for my system) and the productWriter can now complete its task.
I hope this can help anyone who faces a similar issue in the future !
SDB