diff --git a/source/blender/animrig/intern/versioning.cc b/source/blender/animrig/intern/versioning.cc
index ea6ff1be26b..c40ee0f507f 100644
--- a/source/blender/animrig/intern/versioning.cc
+++ b/source/blender/animrig/intern/versioning.cc
@@ -35,6 +35,14 @@ constexpr const char *DEFAULT_VERSIONED_LAYER_NAME = "Legacy Layer";
 
 bool action_is_layered(const bAction &dna_action)
 {
+  /* NOTE: due to how forward-compatibility is handled when writing Actions to
+   * blend files, it is important that this function does NOT check
+   * `Action.idroot` as part of its determination of whether this is a layered
+   * action or not.
+   *
+   * See: `action_blend_write()` and `action_blend_read_data()`
+   */
+
   const animrig::Action &action = dna_action.wrap();
 
   const bool has_layered_data = action.layer_array_num > 0 || action.slot_array_num > 0;
diff --git a/source/blender/blenkernel/intern/action.cc b/source/blender/blenkernel/intern/action.cc
index 97dab7d7c06..2c2489e7fa9 100644
--- a/source/blender/blenkernel/intern/action.cc
+++ b/source/blender/blenkernel/intern/action.cc
@@ -511,6 +511,20 @@ static void action_blend_write(BlendWriter *writer, ID *id, const void *id_addre
 
     const animrig::Slot &first_slot = *action.slot(0);
 
+    /* The forward-compat animation data we write is for IDs of the type that
+     * the first slot is intended for. Therefore, the Action should have that
+     * `idroot` when loaded in old versions of Blender.
+     *
+     * Note that if there is no slot, this code will never run and therefore the
+     * action will be written with `idroot = 0`. Despite that, old
+     * pre-slotted-action files are still guaranteed to round-trip losslessly,
+     * because old actions (even when empty) are versioned to have one slot with
+     * `idtype` set to whatever the old action's `idroot` was. In other words,
+     * zero-slot actions can only be created via non-legacy features, and
+     * therefore represent animation data that wasn't purely from old files
+     * anyway. */
+    action.idroot = first_slot.idtype;
+
     /* Note: channel group forward-compat data requires that fcurve
      * forward-compat legacy data is also written, and vice-versa. Both have
      * pointers to each other that won't resolve properly when loaded in older
@@ -531,6 +545,10 @@ static void action_blend_write(BlendWriter *writer, ID *id, const void *id_addre
   write_slots(writer, action.slots());
 
   if (do_write_forward_compat) {
+    /* Set the idroot back to 'unspecified', as it always should be for layered
+     * Actions. */
+    action.idroot = 0;
+
     /* The pointers to the first/last FCurve in the `action.curves` have already
      * been written as part of the Action struct data, so they can be cleared
      * here, such that the code writing legacy fcurves below does nothing (as
@@ -687,6 +705,12 @@ static void action_blend_read_data(BlendDataReader *reader, ID *id)
     /* Should never be stored as part of the forward-compatible data in a
      * layered action, and thus should always be empty here. */
     BLI_assert(BLI_listbase_is_empty(&action.chanbase));
+
+    /* Layered actions should always have `idroot == 0`, but when writing an
+     * action to a blend file `idroot` is typically set otherwise for forward
+     * compatibility reasons (see `action_blend_write()`). So we set it to zero
+     * here to put it back as it should be. */
+    action.idroot = 0;
   }
   else {
     /* Read legacy data. */