Created attachment 152235 [details]
File produced by OOo2

Insert an arbitrary fontwork from the Fontwork-Gallery.
In the Fontwork toolbar open the "Fontwork Shape" window.
Select any of the types "Arch Left" or "Arch Right".
Notice, that the fontwork text disappears.

The uno-commands, which belong to this icons are:
.uno:FontworkShapeType.fontwork-arch-left-curve
.uno:FontworkShapeType.fontwork-arch-right-curve
.uno:FontworkShapeType.fontwork-arch-left-pour
.uno:FontworkShapeType.fontwork-arch-right-pour

The problem is, that this commands produce a shape type "round-callout". But that shape type is assigned to .uno:CalloutShapes.round-callout,  mso_sptWedgeEllipseCallout and wedgeEllipseCallout. You find it in the "Callout" group of the custom shapes.

So there is a conflict between the draw:type attribute and the draw:text-path attribute for LibreOffice. The conflict is not in ODF, because the type doesn't matter there. But LibreOffice uses the type to track the different kind of shapes for import and export with MS Office.

These four shape types work in OOo2, I have attached such a file. If you open the file in LibreOffice, it reads the contained geometry correctly. But if you assign these types via Icon, LibreOffice tries to use a default geometry, which does not exist. So the resulting shape will be empty.

I have no strong opinion, how to solve the problem. The fontwork-circle-curve and fontwork-circle-pour types, with suitable angles, could be used, for example.