diff --git a/plotly/_subplots.py b/plotly/_subplots.py
index a1bb4219c94..638e08543c0 100644
--- a/plotly/_subplots.py
+++ b/plotly/_subplots.py
@@ -33,10 +33,8 @@
 
 
 def _get_initial_max_subplot_ids():
-    max_subplot_ids = {subplot_type: 0 for subplot_type in _single_subplot_types}
-    max_subplot_ids["xaxis"] = 0
-    max_subplot_ids["yaxis"] = 0
-    return max_subplot_ids
+    # Return a copy to ensure caller can't mutate the shared dict
+    return _INITIAL_MAX_SUBPLOT_IDS.copy()
 
 
 def make_subplots(
@@ -1535,3 +1533,5 @@ def _get_subplot_ref_for_trace(trace):
                 pass
 
     return None
+
+_INITIAL_MAX_SUBPLOT_IDS = {**{subplot_type: 0 for subplot_type in _single_subplot_types}, "xaxis": 0, "yaxis": 0}