I have found 3 citations where it has been extracted from plant material but I would need to know if someone else has found it since in my samples it is in a percentage higher than 20%
I'd say quite a few people have encountered such phthalates at some point. Have you a) stored your plant material in a plastic container or b) has it at any point come in contact with PVC/other plastics? The compound you're talking about is hydrolyzed from Bis(2-ethylhexyl)phthalate, a very common plasticizer used to make plastics softer/more flexible.
Another possibility, without knowing which plant you're talking about, is that your plant has taken up phthalates: phthalates are general air contaminants. However, I don't think phthalates are something your plant should normally contain.