Shiny app not reacting to changes in a Biostrings::DNAString object - r

Question

I'm working on a shiny app that accepts a DNA sequence (e.g. "ACTGACTG"), does some calculations and plots the result when a button is clicked. When I store a Biostrings::DNAString in a reactiveValues object, my shiny app only reacts to changes if the number of characters of the sequence changes, e.g. if "AA" is entered first, the plot doesn't change if "CC" is then entered but does change if "AAAA" is entered. It responds to all changes if I store the object as a character. Here's a simplified example:
library(shiny)
library(shinyBS)
library(Biostrings)
library(ggplot2)
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
ref_seqs <- textInput("ref_seqs", "Sequence", width = "100%",
value = NULL, placeholder = "ATGCTGCTGGTTATTAGATTAGT"),
run_guide <- bsButton("run", 'Run', type = "action",
style = "success", block = TRUE)
),
mainPanel(
plotOutput("reference")
)
)
)
server <- function(input, output) {
ref <- reactiveValues(sq = NULL)
dat <- reactive({
req(input$run)
chrs <- strsplit(as.character(ref$sq),"")[[1]]
data.frame(label = chrs, x = seq_along(chrs))
})
observeEvent(input$run, {
ref$sq <- Biostrings::DNAString(input$ref_seqs)
#ref$sq <- input$ref_seqs
})
output$reference <- renderPlot({
ggplot(dat(), aes(x = x, y = factor(1), label = label)) + geom_text(size = 12)
})
}
shinyApp(ui = ui, server = server)
If I comment out the line ref$sq <- Biostrings::DNAString(input$ref_seqs) and uncomment the line below it, the plot updates upon changes.
Can anyone explain why this happens? Do reactiveValues only work with base types? Thanks!