Bulgaria 1992 1 Lev on undersized blank

I think you are correct.
Weight is close enough.
Planchets start out smaller than the finished coin. The Lev is probably thicker than the Stotinki so there was not enough pressure to expand the planchet.
The reeded edge is applied by the collar die, without enough metal a centered planchet would not expand enough to make contact with the collar and would not be reeded.