Kill everyone, conquer everything? The Philip campaign wasn't designed with diplomacy in mind (was only added in Gold) aside from the objectives, and AI going after sheep was added even later, in, like, patch 1.5.7 or something. Philip campaign is basically blobfest simulator, your units (phalangites and thessalian/barbarian cavalry, not to mention unique units) are so OP, 1vs1 they'll always win unless you send cavalry against hoplites or phalangites against Odrysian peltasts.
So, yeah, no real sense in signing a truce in Philip scenario. Be aware that there is a difference between breaking a truce with your units marching onto enemy territory and breaking the truce formally. Ironically, afair the truce break with your units seems to give less hostility. Save before you break the truce in any case. Or:
Just leave the unit there.