It seems that SNAP only handles mono-swath Cosmo products (both 1st and 2nd generation).
For multi-swath products, only the first one is processed:
Here is the github issue with more details and my workaround at the end: BUG: Missing swaths for multiswaths Cosmo-SkyMed SCS data · Issue #78 · sertit/eoreader · GitHub
Is this a known bug ?