Standalone mode uses temp DB files by default (RIPD-1129):

* If a [database_path] is configured, it will always be used, and tables
  will be upgraded on startup.

src/ripple/app/misc/impl/TxQ.cpp

@@ -1214,7 +1214,7 @@ setup_TxQ(Config const& config)
     set(setup.zeroBaseFeeTransactionFeeLevel,
         "zero_basefee_transaction_feelevel", section);
 
-    setup.standAlone = config.RUN_STANDALONE;
+    setup.standAlone = config.standalone();
     return setup;
 }