If it's a defensive call to arms then breaking the truce wont cause you a stab hit (but you probably know that).
If, as I assume, it's an offensive call, then I agree. This is not a call that the AI (or player in same circumstances) should be able to make. AI certainly should not expect you to honour it. But as it has somehow been able to make the call, you have to bite the bullet & either take a prestige/trust hit to dishonour the call. Or risk a big stab hit & join in.
Playing devil's advocate, the AI may be trying to ditch & damage you as it sees you as an ally it no longer requires. I'm not claiming that the AI is bright enough to be doing this to you for those reasons. But if it is, then you have to applaud its cheek.
Agree that an offensive call to arms should simply not be possible if it would cause an ally an auto stab hit to honour (let alone if they are in significant debt or war exhaustion, or have depleted manpower). As it can be exploited by the player to ditch (& damage) an unwanted ally as described above.
That said this is much more likely a bug in the AI's original calculation to make its DoW, & to then call you to arms. But in any case, it should be reported & addressed by PI.
Suggest you flag it up as a bug report.
