Yes, for the most part
If you knock out a plate legendary, you are greeted with a “try again” message and are brought back to the beginning of the encounter (with all the EXP and stuff you gained from your legendary-killing).
The Vespiquen, however, doesn’t respawn after knocking it out, but it gives you the insect plate anyways. It’s just a Vespiquen; you can find another one if you need it