A Spy’s loadout may also make a huge difference as to how they treat you. Your Eternal Reward (assuming they have a disguise already) makes it considerably easier to pull off a Stab and Sap (i.e. Kill the Engineer first).

Good spies realize that teleporters should be their first order of business unless the enemy is pushed back to their own spawn. While this likely gives the Engineer some warning that there’s a Spy around, the fact that it takes two hits to knock off sappers means that, if you DO knock off the sappers first, they sap your teleporter while you’re busy, cutting off your supply of allies.