I tried the new Metadata-Editor (layer-props) in QGIS 3.0.x and in my case QGIS did not save the metadata with apply/save (tried SHP, GPKG layers) and closing the dialog - only when using "Metadata" - "Save...".
Is this behaviour normal (I think yes) - it is a bit confusing that the metadata does not get saved when using "Apply/Close" (maybe a warning would help)?
Depending on the different file-types (SHP, GPKG/SQLite, PostGIS) it looks like the metadata is always saved to a *.qmd in the project-dir and not in the databases (in case of SQLite/GPKG/PostGIS)?
Is it possible to save the metadata within the databases?