Case & Case list and describe the form, differentiating it from some spurious forms that probably represent mycoplasma infections. In general, I wouldn't expect POWO to track forms, and I think they're pretty harmless to include, when valid if trivial.
POWO doesn't recognize this form, see https://www.inaturalist.org/taxa/357301-Trillium-grandiflorum-roseum/taxonomy_details find to continue deviating but wanted to make sure we're doing so intentionally - any context?