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From WiX Wiki at MindCapers

The reason the TLB is required for .NET assemblies is because the TLB is an installed resource that would need to be tracked in case you let RegistrationHelper create it. The file would for instance be left behind on the system if you don't explicitly remove it when the component is removed. Also I think that if you install an assembly in the GAC you will end up dumping the generated TLB in the Windows\Assemblies directory. Providing the TLB with the installer removes some complexity from the CA's, and also an installation step that might potentially fail.

I have found the problem. I had copied the light command from somewhere else, and it included the -sa switch.

So Light did exactly what I asked it to. Removing the -sa switch meant that the assembly information was generated again.