And then Circuit 1 and Circuit 2 are taken down at the same time because even though they have diverse paths they're part of the same underlying DWDM system which has just encountered a fault.
Ensuring there isn't a single point of failure is not just as simple as 'putting it in the contract.' Carriers can and have groomed their primary and backup circuit on to the same L1 path without realizing it.
That should only happen if your protection is at the optical layer which hasn't been common for a decade or more, partly for that reason.
I've never seen it happen in 20 years. Although of course small parts of the path change over time and sometimes that means bits get put in the same duct for a short run but that also happens when you buy from different providers to be honest. With some carriers you can insist they send you updated route kmls when they change either path and you can detect the change as you'll see different losses/otdr traces before and after a maintenance/fix.
Ensuring there isn't a single point of failure is not just as simple as 'putting it in the contract.' Carriers can and have groomed their primary and backup circuit on to the same L1 path without realizing it.