If by "lens flares" we're referring to the 6 spikey lines jetting from the stars, those are known as diffraction spikes and are a result of the 3 spider vanes holding the secondary mirror in front of the primary mirror cluster at the center of the optical axis. As light enters the primary mirror cluster in the JWT, bright points of lights such as stars have visible aberrations due to the position and orientation of these vanes. In the JWT which uses 3 of them, their orientation creates this 6 spike diffraction pattern. In the case of the Hubble telescope, which is based on the RC telescope design, it uses 4 vanes which creates a 4 spike diffraction pattern instead.