It looks to me like the values are correct, it's just the labels that are over-escaped.
from shiny import reactive
from shiny.express import input, ui, render
ui.input_selectize("x", "Server side selectize", choices=[], multiple=True)
@reactive.effect
def _():
ui.update_selectize(
"x",
choices=[f"Foo & {i}" for i in range(10000)],
selected=["Foo & 0", "Foo & 1"],
server=True,
)
@render.express
def foo():
input.x()
Repro on shinylive
Does NOT appear to be a problem for R. Attempted repro