I agree, i've seen this issue several times. The AI seems able to spontaneously cancel Non Agression pacts, and "Demand tribute" treaties (where they give you money)
In many times i've seen these treaties just mysteriously end, sometimes just 2 turns after they were signed. There's no option to reinstate them, in the diplomacy window they just vanish.
I'm not sure what's causing it, my best guess with the tribute is that they decided to send money to someone bigger and scarier, and that somehow excuses them from an obligation to you. But it doesn't make much sense either way.
Both of these treaties are designed to guarantee a period of peace. If the AI retains the ability to cancel them at will, then these treaties aren't fulfilling their purpose, they don't guarantee anything and are simply means to extract more money from the AI.
On the subject, i do think their durations are a bit long. 50 years is a long time to be in an enforced peace (as is 50 turns), especially since the game offers no recourse for dealing with disputes peacefully (like when your "ally" sticks an outpost between two of your cities and steals a valuable resource)