I've recently purchased a Google Pixel phone, running a vanilla Android 7.1 OS. To preface: I've already contacted Google about this issue, but they say its 3rd party developer territory.
In previous versions of Android (beginning with 4.1 - when I first started using TD), after adding TD as an account with the main Settings (for the OS, not any one app) I've been able to see my TD calendar & contacts within the OS stock Contacts & Calendar app.
Now in 7.1 - that is not possible. Even though TD is listed under Accounts, I cannot add a TD account via the OS Settings> Accounts menu; it appears to freeze, without any errors, though I am able to navigate away from the screen so it's not necessarily "frozen".
CONTACTS: I can view TD contacts within the stock Contacts if I select the appropriate checkbox during the TD setup - but I cannot see the TD account within the stock Contacts settings menu and I cannot select TD as the default location for new contacts.
CALENDAR: I cannot see any TD events, and I cannot see TD within the settings menu for the stock Calendar app.
The work around was simple, add the Exchange account via the stock Email app and select to sync Contacts & Calendar. Now I can see the account in both stock apps. However, I'm sure that this leaves the data I'm syncing more vulnerable than if I were to only use the TD app.
I'm not sure if this is something that the TD developer team and make work, or if this is by design, but I thought to post my experience in case others were wondering about TD and 7.1.