Shiny time slider issue in R - marker icon distortion

I'm having trouble with the impact of a time slider on the circle markers of my interactive shiny/leaflet map. I need circle markers of different color for Fuel_Type (wind & solar), and for radius based of the Nameplate_capacity. Each project also needs to appear on/after its construction start year.

However, when I use the slider the marker size seems to change randomly, meaning some markers are inaccurate for some years, and giving the impression that the site characteristics are changing, when they are not. I have also tried to use addmarkers with custom markers, and the icons seems to change randomly between the projects over the years too.

Here's a portion of my data:

Site_Name   Nameplate_Capacity  Fuel_Type   Construction_Start_Year Latitude Longitude
Beryl Solar Farm    98.4    Solar   2004    -32.35767   149.46648
Wemen Solar Farm    97.5    Solar   2010    -34.81318   142.56044
Karadoc Solar Farm  96.9    Solar   2011    -34.40928   142.25632
Crookwell Wind Farm 96.04   Wind    2015    -34.54083   149.59645
Hallett Stage Brown Hill    94.5    Wind    2007    -33.34865   138.75166
Manildra Solar Farm 94  Solar   2014    -33.17261   148.71472
Bango Wind Farm 240 Wind    2006    -34.56655   148.8488
Sunraysia Solar Farm    228.8   Solar   2008    -34.80279   143.4911
Collector   226.8   Wind    2008    -34.85983   149.39882
Murra Warra Wind Farm   225.7   Wind    2009    -36.43133   142.32266
Limondale Solar Farm    220 Solar   2011    -34.77145   143.50868
Wellington Solar Farm   211.2   Solar   2012    -32.51744   148.95352
Port Augusta Renewable Energy Park  210 Wind    2018    -32.55722   137.84978

and here's the code I used:

library(leaflet)
library(shiny)

#read in data
leafletdf <- read_csv("REsites.csv")

#pop up info
df1 <- leafletdf %>%
  drop_na(Latitude,Longitude) %>%
  mutate(popup_info=paste("<b>Site Name</b>:",Site_Name, "<br/>",
"<b>Capacity:</b>",Nameplate_Capacity,"<br/>","<b>Construction Start Year:</b>", 
Construction_Start_Year))
    
#color pallet
colors <- c("orange","blue")
palette <- colorFactor(colours, df1$Fuel_Type)

#time slider


ui <- bootstrapPage(
  tags$style(type = "text/css", "html, body {width:100%;height:100%}"),
  leafletOutput("map", width = "100%", height = "100%"),
  absolutePanel(bottom = 10, right = 10,
                textOutput("Counter"),
                sliderInput("Year", "Year", min=2000, max=2021, value = 2010, step = 1, sep = "")
  )
)

server <- function(input, output, session) {
  
  sliderData <- reactive({
    df1 %>%
      filter(Construction_Start_Year <= input$Year)
  })
  
  output$Counter <- renderText(
    paste("Number of sites:", nrow(sliderData()))
  )
  
  output$map <- renderLeaflet({
    leaflet(ready) %>%
      addTiles() %>%
      fitBounds(min(df1$Longitude), min(df1$Latitude),
                max(df1$Longitude), max(df1$Latitude))
    
    
  })
  
  observe({
    leafletProxy("map", data = sliderData()) %>%
      clearMarkers() %>%
      addTiles()%>%
      addCircleMarkers(lat = ~Latitude, lng = ~Longitude, stroke = 1, fill=1, opacity = 1, radius=~ sqrt(df1$Nameplate_Capacity/2) , popup =~popup_info, color =~ palette(Fuel_Type))
    
    
    
  })
}


shinyApp(ui = ui, server = server)

Any help would be massively appreciated, thanks for your time!!