It doesn’t have a bright cap. There’s nothing ‘wrong’ with it. They modelled the one they have. A lot of the snarl from a plexi comes from the cap, but also kinda makes it useless for anything other than running it cranked up when it is present.
I remember on the Marshall forum years ago there was a spec spreadsheet of maybe 100 vintage plexis and their specs. They were all over the place, and I’d say the majority had no bright cap. Can probably still find the list.