The absence of any ground net in a project will result in no solder mask on 3D (this means copper traces are exposed).

Although having no ground in a project is very rare, I would still expect the traces should be covered with solder mask.

Project 1: https://www.flux.ai/jharwinbarrozo/no-solder-mask-if-no-ground-fill-in-3d?editor=pcb_3d

Project 2: https://www.flux.ai/jharwinbarrozo/now-with-solder-mask-because-ground-fill-is-present?editor=pcb_3d