If Library/Music font is set to a third-party SMuFL font, editing Library/Line Bodies/Repeatable Symbols and clicking OK resets the music font to Bravura.
I can confirm this, and THANK YOU for figuring this out. A while back I noticed that my font had reverted to Bravura and I had absolutely no clue what caused it so that drove me crazy. But this was it. I normally use Sebastian by the way, so it doesn’t have to be a third party font. Just any font that’s not Bravura.
Sebastian may be bundled with Dorico, but I am definitely not Steinberg. And I’m not you, either, so I guess that makes me some kind of third thing. 
There are a number of aspects where Bravura is a special case (e.g. Clef Changes; default noteheads etc) within Dorico. All those repeatable symbols are font characters, and Bravura is the font that is used for those symbols; but obviously it shouldn’t reset the whole document.
Correct of course, I thought OP might be referring to a font they purchased themselves. But point taken. 
So to rephrase:
It also happens with Petaluma by the way, so it doesn’t have to be a third party font.
Dorico does support clef changes, larger noteheads, and repeatable symbols from non-Bravura SMuFL fonts if they include those glyphs. However, when changing to a non-Bravura font, Dorico will always reset to narrow noteheads. This can be manually corrected in Engraving Options/Notes/Noteheads/Larger Noteheads.
Dorico will always use Bravura for Clef Changes (unless the size for Clef Changes is changed in Engraving Options above a certain size.)
The Larger Noteheads option in Engraving Options always uses Bravura’s noteheads, not the current music font.
That’s what I mean.
This is no longer the case, and Dorico consistently uses the current music font for all clefs; either the regular glyphs or their “small” variants, depending on whether the "clef change scale factor” is above 0.75 or lower.
small variant:
regular clef:
This has also changed, and Dorico has no problem with switching to larger noteheads from a non-Bravura font. See the screenshot below.
