There are two ways to deal with that (at least until the next build or two...), both of which are hacks:
1. Add a geometry field to the table that is a computed field, which generates the centroid for the line. I called the field Centroid and used the expression:
Next, create a drawing from the table using the Centroid field and create a label for that. The result is a point label at the centroid of the line, which often works out OK and has the merit of being horizontal and easily read.
2. Add a geometry field to the table that is a computed field, which generates a straight line from the first coordinate of the line to the last coordinate of the line. I called the field Segment and used the expression:
GeomMakeSegment(GeomCoordXY([Geom], 0), GeomCoordXY([Geom], GeomCoordCount([Geom])-1))
You can then create a drawing from the table using that Segment field, and make labels from that drawing. Since the lines will be roughly in the same orientation as the curvy line you get a tilted label.
Neither approach, of course, is as nice as a curvy label that follows a curved line. But... I use the first trick quite a bit since I like the legibility of horizontal labels. It can also be useful to create the centroid geoms using the transform panel, that is, not as computed geoms, so then you can drag around the labels as you like, for greater freedom and control.