Only certain plot points responding to row selection in shiny app?

I am attempting to adapt an example (2.1.1) from the tutorial found here on interactive plots in shiny. I have a shiny app as follows:

Data:

seats = data.table(
ID = c("1","2","3","4","5","6"),
Row = c("A", "A", "A", "B", "B", "B"),
SeatNum = c(1,2,3,1,2,3),
y = c(1,1,1,2,2,2),
price = 45)

ui.R

fluidPage(

title = 'Select Table Rows',

h1('A Client-side Table'),

fluidRow(
    column(6, DT::dataTableOutput('x1')),
    column(6, plotOutput('x2', height = 500)),
    column(3, verbatimTextOutput('x4')),
    column(4, verbatimTextOutput('x5'))
),

hr(),

)

server.R

shinyServer(function(input, output, session) {

output$x1 = DT::renderDataTable(seats, editable = "row", server = FALSE)

# highlight selected rows in the scatterplot
output$x2 = renderPlot({
    s = input$x1_rows_selected
    par(mar = c(4, 4, 1, 1))
    plot(seats$SeatNum, seats$y)
    if (length(s)) points(seats[s, , drop = FALSE], pch = 19, cex = 2)
})

output$x4 = renderPrint({
    s = input$x1_rows_selected
    if (length(s)) {
        cat('Combined price \n of all seats:\n\n')
        cat(sum(seats[s,]$price))
    }
})

output$x5 = renderPrint({
    s2 = input$x1_rows_selected
    if (length(s2)) {
        cat('Total number of seats selected:\n\n')
        cat(length(s2))
    }
})

})

Upon launching the app I can select any of the first three rows and the plot reacts appropriately. However, from row 4 onwards the plot does not respond. I've played around with the if (length(s)) points(seats[s, , drop = FALSE], pch = 19, cex = 2) line but I don't understand its behavior.