Possibly. If you cashed out a policy that had investment value, the value of the investment up to the date of death is not taxable, but any increased value between the date of death and the date when you cashed out the policy is taxable income to you. Similarly if you received a death benefit, that is not taxable. But if the company added interest to the death benefit counting from the date of death to the date the benefit was paid, that interest is taxable to you.
It might be easier to understand with a simpler investment like stocks. If you inherit stocks and they are $100 a share on the date the person died, and you sell them at $102 per share, your taxable income is $2 per share (the amount they increased in value after the person died). Or, if they paid a dividend before the prior owner died, that money is not taxable to you, but if they paid a dividend after the previous owner died, the income is taxable to you, even if you didn't have time to legally transfer ownership before the dividend was paid.
If you got a 1099-INT it is probably interest that was credited after the inheritance was issued. If that's the case, it will be taxable.
Missing a document happens frequently, and there is no reason to panic.
If you don't file an amended return,
If you choose to prepare an amended return you will have to wait until your originally filed return is fully processed and then amend your return to add the missing document.