Reprojection of the small scale Natural Earth coastline dataset to a Robinson projection returns an erroneous line feature on the plot. (Apologies if this is the wrong place to report, but it appears to be an issue with coord_sf rather than the Natural Earth vector dataset).

Medium-scale coastlines reproject okay to a World Robinson projection (EPSG:54030)
coastlines <- ne_coastline(scale = "medium", returnclass = "sf")
ggplot(data = coastlines) + geom_sf(colour = "black") + coord_sf(crs = st_crs(54030)) + theme_bw()

Small-scale coastlines display okay at the default projection
coastlines <- ne_coastline(scale = "small", returnclass = "sf")
ggplot(data = coastlines) + geom_sf(colour = "black") + theme_bw()

Small-scale coastlines reprojected to the World Robinson projection (EPSG:54030) include the erroneous line feature at ca. 70 degrees north
coastlines <- ne_coastline(scale = "small", returnclass = "sf")
ggplot(data = coastlines) + geom_sf(colour = "black") + coord_sf(crs = st_crs(54030)) + theme_bw()