Can't join coordinates to polygons represented by black icons in RStudio viewer

I’m using {sf} to import a GeoJSON file. What does the black icon represent? I’m trying to join coordinates to this object via st_join(ken_con_sf, join = st_within) and it seems to fail where coordinates fall within the polygons represented by these icons.

library(sf)
ken_con_sf <- 
    read_sf("https://raw.githubusercontent.com/mikelmaron/kenya-election-data/master/data/constituencies.geojson") %>%
    st_transform(4326)

R version 4.0.2 (2020-06-22)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Catalina 10.15.7

Matrix products: default
BLAS:   /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] maps_3.3.0           sp_1.4-2             sf_0.9-6            
 [4] ggrepel_0.8.2        zoo_1.8-8            leaflet_2.0.3       
 [7] showtext_0.9         showtextdb_3.0       sysfonts_0.8.1      
[10] lubridate_1.7.9      shinyWidgets_0.5.3   rmarkdown_2.5       
[13] shiny_1.5.0          flexdashboard_0.5.2  dplyr.teradata_0.3.2
[16] niviR_0.1.0          dbplyr_1.4.4         forcats_0.5.0       
[19] stringr_1.4.0        dplyr_1.0.2          purrr_0.3.4         
[22] readr_1.4.0          tidyr_1.1.2          tibble_3.0.4        
[25] ggplot2_3.3.2        tidyverse_1.3.0      RSQLite_2.2.1       
[28] RPostgres_1.2.0     

loaded via a namespace (and not attached):
 [1] fs_1.5.0           bit64_4.0.5        httr_1.4.2        
 [4] tools_4.0.2        backports_1.1.10   utf8_1.1.4        
 [7] R6_2.5.0           KernSmooth_2.23-17 DBI_1.1.0         
[10] colorspace_1.4-1   withr_2.3.0        tidyselect_1.1.0  
[13] bit_4.0.4          compiler_4.0.2     cli_2.1.0         
[16] rvest_0.3.6        xml2_1.3.2         bookdown_0.21     
[19] scales_1.1.1       classInt_0.4-3     odbc_1.2.3        
[22] digest_0.6.27      pkgconfig_2.0.3    htmltools_0.5.0   
[25] fastmap_1.0.1      htmlwidgets_1.5.1  rlang_0.4.8       
[28] readxl_1.3.1       keyring_1.1.0      rstudioapi_0.11   
[31] generics_0.1.0     jsonlite_1.7.1     crosstalk_1.1.0.1 
[34] magrittr_1.5       Rcpp_1.0.5         munsell_0.5.0     
[37] fansi_0.4.1        lifecycle_0.2.0    stringi_1.5.3     
[40] yaml_2.2.1         grid_4.0.2         blob_1.2.1        
[43] promises_1.1.1     crayon_1.3.4       lattice_0.20-41   
[46] haven_2.3.1        hms_0.5.3          knitr_1.30        
[49] pillar_1.4.6       reprex_0.3.0       glue_1.4.2        
[52] evaluate_0.14      modelr_0.1.8       vctrs_0.3.4       
[55] httpuv_1.5.4       cellranger_1.1.0   gtable_0.3.0      
[58] assertthat_0.2.1   xfun_0.19          mime_0.9          
[61] xtable_1.8-4       broom_0.7.0        e1071_1.7-3       
[64] later_1.1.0.1      rsconnect_0.8.16   class_7.3-17      
[67] memoise_1.1.0      units_0.6-7        ellipsis_0.3.1