diff --git a/lib/microwaveprop_web/live/rover_planning_live/form.ex b/lib/microwaveprop_web/live/rover_planning_live/form.ex
index 48f5e87c..899c51d6 100644
--- a/lib/microwaveprop_web/live/rover_planning_live/form.ex
+++ b/lib/microwaveprop_web/live/rover_planning_live/form.ex
@@ -217,6 +217,7 @@ defmodule MicrowavepropWeb.RoverPlanningLive.Form do
         </div>
 
         <label class="flex items-center gap-2 cursor-pointer">
+          <input type="hidden" name="mission[only_known_good]" value="false" />
           <input
             type="checkbox"
             name="mission[only_known_good]"
@@ -224,7 +225,6 @@ defmodule MicrowavepropWeb.RoverPlanningLive.Form do
             checked={Phoenix.HTML.Form.normalize_value("checkbox", @form[:only_known_good].value)}
             class="checkbox checkbox-primary"
           />
-          <input type="hidden" name="mission[only_known_good]" value="false" />
           <span>
             Only check against known good locations
             <span class="text-xs opacity-70 block">
diff --git a/test/microwaveprop_web/live/rover_planning_live_test.exs b/test/microwaveprop_web/live/rover_planning_live_test.exs
index e819c1ce..994478e9 100644
--- a/test/microwaveprop_web/live/rover_planning_live_test.exs
+++ b/test/microwaveprop_web/live/rover_planning_live_test.exs
@@ -83,6 +83,31 @@ defmodule MicrowavepropWeb.RoverPlanningLiveTest do
                live(conn, ~p"/rover-planning/new")
     end
 
+    test "typing into the form does not flip the only_known_good checkbox", %{conn: conn} do
+      user = AccountsFixtures.user_fixture()
+      conn = log_in_user(conn, user)
+      {:ok, lv, _html} = live(conn, ~p"/rover-planning/new")
+
+      html =
+        lv
+        |> form("#mission-form",
+          mission: %{
+            name: "Some name",
+            band_mhz: "10000",
+            rover_height_ft: "8.0",
+            station_height_ft: "30.0",
+            only_known_good: "true",
+            stations: %{"0" => %{input: ""}}
+          }
+        )
+        |> render_change()
+
+      # The default-checked box must STILL be checked after a phx-change.
+      # Hidden "false" + checkbox "true" with hidden first = checkbox wins.
+      assert html =~
+               ~s(name="mission[only_known_good]" value="true" checked)
+    end
+
     test "logged-in user can create a mission with a callsign-input station",
          %{conn: conn} do
       user = AccountsFixtures.user_fixture()