I haven't used asciidoctor-diagram, but the lesson from the previous issue is: make sure the glyphs are present in the font that is being used. Open the font in FontForge and check that the glyph your are trying to render exists.
I agree, the fonts probably misses the corresponding glyphs.
However I don't know which font is used. It is not the one that is used for the rest of the document where the diagram is embedded In the rest of the document all umlauts are shown correctly.
Seems like diagram seems to take another font (why and from where I don't know).
For me it's interesting to know if anyone can tell me how to change the font parameter in the Asciidoc file.