Fix STL inverse transform bug (#4328)

* Switch old_seasonal to old_seasonality in inverse_transform * release notes
alteryx · Oct 4, 2023 · 6b291b6 · 6b291b6
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commit 6b291b6
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diff --git a/docs/source/release_notes.rst b/docs/source/release_notes.rst
@@ -10,6 +10,7 @@ Release Notes
         * Extended TimeSeriesRegularizer to support multiseries :pr:`4303`
     * Fixes
         * Fixed forecast period generation function for multiseries :pr:`4320`
+        * Fixed bug in ``STLDecomposer.inverse_transform`` causing incorrect seasonality projections :pr:`4328`
     * Changes
         * Updated ``split_data`` to call ``split_multiseries_data`` when passed stacked multiseries data :pr:`4312`
         * Pinned pandas version under 2.1.0 :pr:`4315`

diff --git a/evalml/pipelines/components/transformers/preprocessing/stl_decomposer.py b/evalml/pipelines/components/transformers/preprocessing/stl_decomposer.py
@@ -380,10 +380,12 @@ def inverse_transform(
             if len(y_t.columns) > 1:
                 old_trend = self.trends[id]
                 old_seasonal = self.seasonals[id]
+                old_seasonality = self.seasonalities[id]
                 period = self.periods[id]
             else:
                 old_trend = list(self.trends.values())[0]
                 old_seasonal = list(self.seasonals.values())[0]
+                old_seasonality = list(self.seasonalities.values())[0]
                 period = list(self.periods.values())[0]
             # For partially and wholly in-sample data, retrieve stored results.
             if index[0] <= series_y.index[0] <= index[-1]:
@@ -423,7 +425,7 @@ def inverse_transform(
                 ) = self._project_trend_and_seasonality(
                     truncated_y_t,
                     old_trend,
-                    old_seasonal,
+                    old_seasonality,
                     period,
                 )