I am using QGIS 3.10 to display American Community Survey variables at the census tract level. Last week, I successfully joined three ACS tables to duplicated copies of census tract geometries (same source), styled them, printed maps using the style, saved the project, and closed the application.
Today, I reopened the project to find that the original attributes from the joined attribute tables are no longer in the expected fields, and most geometries were initially not displayed in the map viewer. However, two attributes derived using field calculator remained in a clean row/column format, and the geometry seems ok, because all expected shapes were shown when reclassifying by one of the derived variables.
Both files are encoded in UTF-8, and no changes were made to the files between the last time I used them and present. Several other duplicate copies of the shapefile joined to different census tables were not affected. Is it apparent from this description what happened here, and if so, how to keep it from happening in the future?