tm_scale_continuous displaying with interval legend with linestrings

[marine-ecologist]

I'm converting v3 code to v4 and can't seem to get the new tm_scale_continuous to work with linestrings in that the continuous legend color scale reverts to intervals. Example below:

# generate 20 linestrings  with numeric groups:

library(sf)

set.seed(42)

lon_min <- -180
lon_max <- 180
lat_min <- -90
lat_max <- 90

create_random_line <- function() {
  lon_start <- runif(1, lon_min, lon_max)
  lat_start <- runif(1, lat_min, lat_max)
  lon_end <- runif(1, lon_min, lon_max)
  lat_end <- runif(1, lat_min, lat_max)
  
  st_linestring(matrix(c(lon_start, lat_start, lon_end, lat_end), ncol = 2, byrow = TRUE))
}

lines <- lapply(1:20, function(x) create_random_line())

lines_sf <- st_sfc(lines, crs = 4326)  # Use WGS84 (EPSG:4326)
values_df <- data.frame(value = 1:20)
lines_sf <- st_sf(values_df, geometry = lines_sf)

library(tmap)

tm_shape(lines_sf) +
  tm_lines("group", 
           lwd=5, 
           tm_scale_continuous(values="brewer.spectral")) +
tm_place_legends_right()

When changing to points tm_scale_continuous works fine (presumably as it's fill and not col?):

tm_shape(points_sf) +
  tm_dots("value", 
           size=2, 
           tm_scale_continuous(values="brewer.spectral")) +
tm_place_legends_right()

is there a way of getting a continuous colorscale legend with col?