If you talk about the math font, I do not see what you mean. Daniel
has worked a lot on his fonts, and they now work well.
indeed the body font is not concrete opentype math but it looks very similar at first sight
(of course... ). I was deceived because I remember its shapes quite well (C ,g a ).
If you talk about the text font, Hans changed the concrete typescript
so that it uses the variable "mono" Latin modern instead of the
concrete text font that comes from Computer modern unicode. But the
font is also boldened slightly, and perhaps that is what causes your
problem (It did look weird on my phone for half a second or so before
it "got normal"). In okular I see no problems.
here evince & mupdf are quite ok (evince somewhat bolder than mupdf), okular looks not so good. They all have a caching-something,
after the first zooms the pdf is rendered differently.
--
luigi