Merge pull request #15680 from edx/ormsbee/readd_to_bt_cache_on_miss

Re-add to BlockStructure cache on miss.

openedx/core/djangoapps/content/block_structure/store.py

@@ -93,6 +93,7 @@ class BlockStructureStore(object):
             serialized_data = self._get_from_cache(bs_model)
         except BlockStructureNotFound:
             serialized_data = self._get_from_store(bs_model)
+            self._add_to_cache(serialized_data, bs_model)
 
         return self._deserialize(serialized_data, root_block_usage_key)