Distillery hot-update for long running processes not working

I’ve been testing how hot updates behave for long running GenServers, but so far, the GenServer process is being killed whenever I upgrade. I have tried setting a higher :shutdown and adding Process.flag(:trap_exit, true) to the init callback function, but still without success.

Some relevant parts of my GenServer:

use GenServer, shutdown: 120 * 1000

  @doc false
  def start_link(_opts) do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end

  def run_testapp do
    GenServer.call(__MODULE__, {:run_testapp}, 60 * 1000)
  end

  @doc false
  @impl true
  def init(:ok) do
    Process.flag(:trap_exit, true)
    :ok
  end

  @doc false
  @impl true
  def handle_call({:run_testapp}, _from, state) do
    Process.sleep(24 * 1000)
    {:reply, %{a: 1}, state}
  end

Upgrades work fine if the process is not running, but otherwise the running process is killed and I get

** (exit) exited in: GenServer.call(Testapp.Server, {:run_testapp}, 60000)
    ** (EXIT) killed
    (elixir 1.10.0) lib/gen_server.ex:1023: GenServer.call/3
iex(app@127.0.0.1)5> Testapp.Server.run_testapp

Any ideas or resources that could help me?