You are confusing Materials with Material Slots
Material slots are part of the mesh data and hold information about which face gets assigned which material
Materials hold the actul surface shading data (color, roughness, reflectivity, emission, etc.)
Material slots can't really be named and only hold face data.
In your case your mesh has several material slots where you can assign materials, but only one (the one with the name) really has one material assigned, the others are empty.
You most likely lost your materials when joining meshes and used as 'master mesh' an object without materials.
To get them back either go to an earlier version of the model (if you kept any saved) or try the backup file ##.blend1 or ##.blend2 if haven't saved over it yet, otherwise it will no longer hold a version with materials.