hovermode='x unified' is causing unexpected behavior together with group_by()

library(tidyverse)
library(plotly)

TexasCities_plus_Houston <- ggplot2::txhousing %>% 
  group_by(city) %>%
  plot_ly(x = ~date, y = ~median) %>%
  add_lines(
    name = "Texan Cities", 
    text = ~paste0(city, "\n", median),
    hoverinfo = "text+x"
  ) %>%
  filter(city == "Houston") %>%
  add_lines(name = "Houston")

This works as expected (but the limitation is that it is hard to find Houston city to hover on):
TexasCities_plus_Houston %>% layout(hovermode = "closest")

This solves the above limitation, however, it does not matter which point you hover on, the same city name is shown ("Wichita Falls"). The "median" variable changes as expected, the problem seems to be solely with the variable used in group_by(), in this case, "city".
TexasCities_plus_Houston %>% layout(hovermode = "x unified")

まず、plot_ly()、group_by(city)、add_lines()、および layout(hovermode = "x unified") を使って、提供された R の再現コードを実行します。統合されたホバーラベルを hovermode = "closest" と比較し、グループ化された city の値がどのように選択されるかを追跡します。完了の条件は、統合されたホバーに、変化する中央値を維持しながら、ホバーした点に属する city が表示されることです。