Why am I frequently prompted to sign in the add-in?

You will be prompted to sign in only if your sign-in token validity has expired. Let’s see what this means.

After your first sign-in, the add-in captures and stores the sign-in token and ensures that the token is valid for the next 48 hours. Whenever you open the add-in within 48 hours, your sign-in token’s validity is automatically extended for the next 48 hours. This way, as long as your next session on the add-in is within 48 hours, the add-in won’t prompt you to sign in.

But, in situations such as you had a long weekend or you went on a vacation, you might be opening the add-in only after 48 hours from your last sign-in time. By that time, your token validity expires. In such situations, the add-in prompts you to sign in again.