There is a empty space on the map. The data is ivalable for those regions. What could be the Problem?
Geo Data for Germany:
URL <- "https://biogeo.ucdavis.edu/data/gadm3.6/Rsp/gadm36_DEU_1_sp.rds"
germany <- readRDS(url(URL))
germany <- fortify(germany)
df_Karte_ratio %>% 
  ggplot(aes(x = long, y = lat, 
                              group = group, fill = ratio_fl_arzt
                              ))+
  geom_polygon(col = "black") +
  coord_map()+
  facet_grid(~EP01_KJP02_BEIDE03_PSM_04_20)+
  labs(title = "Fälle-Arzt Verhältnis")+
  theme_map()+
  theme(
    legend.position  = "right",
    #legend.direction = "horizontal",
    legend.title = element_blank())+
   scale_fill_continuous_sequential(palette = "BluYl")
Sadly I could not find any information about this issue

 
    