Hello; I happen to think live plants can be a source of some bb. I have no proofs for this notion. I get that the new growth such as plant leaves likely will not have much chance of holding bb, the older growths and parts of a plant may house the bb as a bio-film.I've read getting live plants in pot (with the wool around the roots) could be a good source of live bacteria? Have you ever heard this?
Do plants have repellents or other mechanisms to shed bacteria from the surfaces? I see this as a possibility as plants are chemical factories so some could have such abilities. I am not convinced the more common aquarium can do this.
Another thing that fostered this idea for me has been the nitrogen fixing bacteria associated with the roots of legumes.
At any rate the plants you describe might have some bb on the wrapped base.