How to ensure sending a .ICS file on Gmail will result in an "add to calendar"

I have noticed that sometimes, when I receive a calendar invite (.ICS file) I do not get an add to calendar option in my Gmail read view.

Notice no add to calendar option.

Other times I do get this:

Here we do have the option.

So, when sending an email, how do I ensure Gmail detects the calendar .ICS file is attached and offers the user an add to calendar option?