library(tmap)
library(tmaptools)

# works fine
tm_shape(NLD_dist) +
  tm_polygons(
    fill = "edu_appl_sci",
    col = NULL) +
  tm_shape(NLD_muni) +
  tm_borders() +
  tm_title("Amsterdam", size = 0.8, position = c("left", "center")) +
  tm_inset(bb("Amsterdam"), position = c("left", "center")) 

# overlaps
tm_shape(NLD_dist) +
  tm_polygons(
    fill = "edu_appl_sci",
    col = NULL) +
  tm_shape(NLD_muni) +
  tm_borders() +
  tm_inset(bb("Amsterdam"), position = c("LEFT", "CENTER")) +
  tm_title("Amsterdam", size = 0.8, position = c("LEFT", "CENTER"))