Omit not working when interface has index signature

Question

Running the latest version of Typescript (4.6.4) when my interface has an index signature Omit doesn't work as expected. Not sure if this is expected behavior or I am misunderstanding something with index signatures.

interface Test1 {
  a: number;
  b: number;
}

// 'Property 'b' is missing in type '{}' but required in type 'Omit<Test1, "a">''
const test1: Omit<Test1, 'a'> = {};

interface Test2 {
  [key: string]: number;
  a: number;
  b: number;
}

// no issues
const test2: Omit<Test2, 'a'> = {};

I would expect that test2 would have the same error as test1.