clusterOptions() displaying correct area of map but no markers or clusters

Question

I am working on a Shiny app in R that creates a leaflet map and plots points.

Plotting the points by themselves works fine. The issue arises when I try to implement even the most basic clusters.

I can plot individual points with no problem. As soon as I try to implement clusterOptions=markerClusterOptions(), app runs and displays a map with no markers or clusters.

I created simplified data to troubleshoot. This code below creates a data frame with 9 points. They're intentionally created to display as three sets of three points. They display correctly.

test_data <- data.frame(
        longitude = c(-90, -91, -90.5, -90.01, -91.01, -90.51, -90.02, -91.02, -90.52),
        latitude = c(45, 45.5, 45.25, 45.01, 45.51, 45.26, 45.02, 45.52, 45.27)
      )
      
      leaflet(data = test_data) %>%
        addProviderTiles(providers$CartoDB.Positron) %>%
        addCircleMarkers(
          lng = ~longitude, lat = ~latitude,
          radius = 5  # Fixed radius for testing

        )

I then try to add in basic clustering, which should give me 3 clusters on the same map. App runs, map displays the same area, but no points or clusters.

 test_data <- data.frame(
        longitude = c(-90, -91, -90.5, -90.01, -91.01, -90.51, -90.02, -91.02, -90.52),
        latitude = c(45, 45.5, 45.25, 45.01, 45.51, 45.26, 45.02, 45.52, 45.27)
      )
      
      leaflet(data = test_data) %>%
        addProviderTiles(providers$CartoDB.Positron) %>%
        addCircleMarkers(
          lng = ~longitude, lat = ~latitude,
          radius = 5,  # Fixed radius for testing
          clusterOptions=markerClusterOptions() # This should create 3 default clusters. 
        )

I've updated leaflet, RStudio, etc. No change, no errors being thrown in RStudio.

Update:

I've confirmed that the issue is not my underlying Leaflet package. This barebones code correctly displays some basic clusters of points on a U.S. map.

library(leaflet)
library(leaflet.extras)

# Create a sample data frame with 9 points in the U.S.
data <- data.frame(
  latitude = c(39.8283, 34.0522, 41.8781, 29.7604, 32.7157, 33.4484, 36.7783, 30.2672, 40.7128),
  longitude = c(-98.5795, -118.2437, -87.6298, -95.3698, -117.1611, -112.0740, -119.4179, -97.7431, -74.0060),
  group = c("Group A", "Group B", "Group A", "Group C", "Group B", "Group A", "Group C", "Group B", "Group A")
)

# Create the Leaflet map with basic marker clusters
leaflet(data) %>%
  addTiles() %>%
  addCircleMarkers(
    lng = ~longitude, lat = ~latitude,
    clusterOptions = markerClusterOptions(),
    color = ~group,
    radius = 8
  )```

Answer 1

Found the issue.

I was setting a custom leaflet version, which was older than the default I have on my machine. Once I removed the custom leaflet map displayed with clusters.