Strange isolation level being set to "read committed"

[variable]

"mssql": {
           "ENGINE": "sql_server.pyodbc",
           "NAME": "...",
           "USER": "...",
           "PASSWORD": "....",
           "OPTIONS": {
                "dsn": "...",
                "initial_state": {
                    "quoted_identifier": "on",
                    "concat_null_yields_null": "on",
                    "ansi_warnings": "on",
                    "ansi_nulls": "on",
                    "ansi_padding": "on",
                    "implicit_transactions": "off"
                },
                "extra_params": "tds_version=7.3",
                "isolation_level": "READ UNCOMMITTED"
           },
          "TEST": {"MIRROR": "default"}
        }

I have the following settings with "isolation_level" set to `READ UNCOMMITTED", but every now and then we have query stuck with isolation level set to "READ COMMITTED", anyone having similar issue?