Well, that certainly is different from my experience.

I've ivated an alliance as Julii with both Pontus and Scythia, and it has lasted for nearly 20 years (40 turns), even though I border both of them. I hold 83 territories and Pontus holds 4 and Scythia holds 5. Maybe you're doing something to offend your allies. I've also had an alliance with the Macedonians against the Greek Cities fall apart when I left my territories bordering Macedon too poorly garrisoned.
But having learned that lesson, I DON'T rely on my allies to be faithful. I garrison large armies on my borders with them, on the highways between our cities. And voila, no backstabbing.
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