Why mathematical expressions not working in panel.labels argument of tm_layout() in tmap v.4?

I am trying to have mathematical expression in the panel labels. It was working with tmap version 3. I am using the following code:

library(tmap)
library(terra)
library(sf)

r <- rast(system.file("ex/elev.tif", package="terra"))

states <- st_read(system.file("ex/lux.shp", package="terra"))

# make some bbox magic
bbox_new <- st_bbox(r) # current bounding box

xrange <- bbox_new$xmax - bbox_new$xmin # range of x values
yrange <- bbox_new$ymax - bbox_new$ymin # range of y values

bbox_new[1] <- bbox_new[1] - (0.05 * xrange) # xmin - left
bbox_new[3] <- bbox_new[3] + (0.05 * xrange) # xmax - right
bbox_new[2] <- bbox_new[2] - (0.05 * yrange) # ymin - bottom
bbox_new[4] <- bbox_new[4] + (0.05 * yrange) # ymax - top

bbox_new <- bbox_new %>%  # take the bounding box ...
  st_as_sfc() # ... and make it a sf polygon


tm_shape(r, bbox = bbox_new) +
  tm_raster(col.scale = tm_scale_continuous(
    values = "viridis"),  # color palette; 
    col.legend = tm_legend(title = "Legend", frame.lwd = 0,
                           position = c("LEFT", "BOTTOM"))) +
  tm_shape(states) +
  tm_borders() +
  tm_layout(panel.labels = expression(bold("Elevation (No. m"^-2*")"))) #Fake unit just to show

which returns me

Error in bold("Elevation (No. m"^-2 * ")") : could not find function "bold"

How to have superscripts and subscripts in panel labels using tmap v.4?

With the development version of tmap it is working fine. The development version can be installed from the GitHub repository using remotes and pak packages or from the R-universe repository.

# install.packages("remotes")
remotes::install_github("r-tmap/tmap")

# install.packages("pak")
pak::pak("r-tmap/tmap")

# Or from r-universe
install.packages("tmap", repos = c("https://r-tmap.r-universe.dev", "https://cloud.r-project.org"))