Since I changed to Solidworks 2021, I have parts that randomly switch to wireframe in the assembly. To correct it, I have to save and close the assembly, then re-open. I have checked the display settings in both the assembly and part level. All seem to be correct. I do not recall this happening in any version prior to 2021. Nothing in my hardware has been changed.
We have this same issue on several workstations on 2021 SP1.0. We updated from 2018 and no changes on hardware having these issues. We have not yet found a solution.