format and behaviour op drop-down lists for custom metadata in outliner

These drop-down lists (ddl’s) behave differently from the ddl’s for e.g. ‘Label’ or 'Status. They seem to take up all the space between items in the outline. This looks odd. BUT, and this is more serious: when opened, the selected item allways reverts to the 1st one on the list (‘geen’, i.e. ‘none’), thereby forgetting the current selection.
In this example, the test-metadata for section ‘De Teen’ was ‘foo’ when I opened the ddl. Metadata automagically reverted to ‘geen’. This is not as expected. Also, after selecting an item from the ddl, the ddl stays visible until I click somewhere else. This behaviour is also different than that of the other ddl’s.
Best wishes for 2021,