split function doesn't work without from

SELECT upper('foo') AS a /* OK */;
+-----+
|  a  |
+-----+
| foo |
+-----+

SELECT split('foo,bar,buz',',') AS a /* NOK */;
Error: PLAN ERROR: Failure while materializing expression in constant expression evaluator [SPLIT('foo,bar,buz', ',')].  Errors:
Error in expression at index -1.  Error: Only ProjectRecordBatch could have complex writer function. You are using complex writer function split in a non-project operation!.  Full expression: --UNKNOWN EXPRESSION--.

SELECT split(a,',') AS a FROM (SELECT 'foo,bar,buz' AS a) /* OK */;
+---------------------+
|          a          |
+---------------------+
| ["foo","bar","buz"] |
+---------------------+