That’s a really nice observation. If you look at images of JWST you can see that we are mainly seeing the 21x14m sunshield but there is other stuff on the sunward side of the spacecraft including the solar panel and the spacecraft bus and propulsion units. Some sunlight will be scattered back to us from these other components but there will be strong glints from the sunshield which will be highly dependent on the relative angle between the sun and the observer. Given JWST’s configuration even small changes in this angle will make a significant change to the magnitude we see.
When the telescope is operating at L2 it is in a large radius halo orbit and the attitude will change for each observation since the telescope and sunshade are fixed to each other. It will be an interesting project to try to predict when glints would occur given the target list that the telescope will be aimed at.