Dorico previews any font that has SMuFL metadata as though it’s a music font: it should show a treble clef, a bass clef, an eighth/quaver and a trill sign.
Dorico previews regular text fonts with the name of the font and size.
I don’t know why MakeMusic attached SMuFL stuff to its text fonts, and I don’t own the Groove fonts so can’t check them, but redundant SMuFl gubbins is likely the issue: if a font is a plain text font it won’t contain clefs, notes or ornaments in the locations SMuFl defines, so the preview will show gibberish.
It shouldn’t have any impact on your actual use of the fonts within Dorico, as long as you’re using music fonts where music fonts are expected, and text fonts where text fonts are expected.
If you’re telling me that there’s nothing to worry about, then I won’t worry too much about it, although how do I check and get rid of the redundant SMuFL?
Click the name or avatar in his message. That will take you to a panel with a Message button as long as you have been on the forum long enough to reach that trust level, which usually comes pretty early in a users time here.
When I put in cord symbols, it doesn’t show the flats or sharps anymore when I’m using any of the third party fonts (Bb shows up as B, or F# comes up as F). Also, the rhythm slashes have disappeared.
Last thing, when using Bravera, all I get is a bunch of letters where there should be notes.
Which fonts are you using, and for which Font Style? Remember that some styles need to be “Music” fonts, like Bravura, and some need to be Text fonts.
If you’ve selected a text font for something that needs to display accidentals, then that usually won’t work, because text fonts don’t usually have accidental symbols.