[lxdude]

I mean I understand how the shutter works. I've got a few other cameras that use a similar vertical curtain (Yashica TL Electro X, Voigtlander R2A, my DSLRs, Nikon N80)

Just none of them do what you're describing. They all open up a full white frame at 1/1000th.

No, they don't, unless there's something wrong with the shutter. They may appear to be a full white frame, but at 1/1000th the shutter is a fairly narrow slit. Using a flash on single burst will verify this.

Except my Pentax Spotmatic which also does not work at 1/500th and 1/1000th. It has a horizontal cloth shutter though, but the effect is the same.

It will be with any focal plane shutter above maximum sync. speed.