Unfortunately, as you’ve discovered, the Edit Chord Symbol Component window is very buggy and takes a lot of trial and error to accomplish anything once you have multiple glyphs involved. I’m not sure why Dorico won’t show the flats, but my guess is that you have globally edited that composite character somehow. Once you enter the Edit Chord Symbol Component window you are making a global change to that suffix that will be reflected wherever it appears in your score. Changes made in the Edit Chord Symbol Appearance window are local.
“b13” is actually a composite character called comp.csac.smufl.glyph.alteration.prefix.flat.13. If you made a global adjustment to your “b13” and “b9” suffixes in the Edit Chord Symbol window, that could possibly explain why the flats aren’t showing. If you want to post a file, I’m sure someone could help diagnose the issue.
Here’s where to find that character if you want to rebuild the chord suffix:
