Probably not a bug, but cache corruption, which happens rarely. It's good practice to clear the cache prior to beginning a new project.
Don't clear it if simply reloading the last project worked on or the cache will have to be rebuilt.
Clear Unused Cache Files can be used at anytime during editing. Save time by not loading files that won't be used in the project.
For protection, Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar.
The latest 64-bit version, recommended for lengthy, UHD and complex projects, is linked above. The home download page is here.