I don't really know what I can tell you- in the testing I've done to try to recreate this and my own playing, i haven't noticed them being anything like that unless it's a bug that was occurring in release builds for some reason that is no longer there. There's also nothing in their stats, armour class, size class, rig, etc that would be consistent with it, either.
Did some testing and it seems the IPV is the only AA corvette that has issues. None of the others that I can observe have issues with indestructibility, republic or imperial they blow up very quickly when under fire. So it's just the IPV that has issues and needs to be fixed. They don't appear to be smaller than the Corellian corvette and those things go down quickly. So again I'll say there's something up with the IPV and it needs to be addressed.
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