Google BigQuery - Bug using WITH and RAND()

Question

I came upon an interesting behaviour when I use RAND() within WITH:

WITH
  t0 AS ( SELECT rand() AS c ),
  t1 AS ( SELECT c FROM t0 ),
  t2 AS ( SELECT c FROM t0 )

SELECT c FROM t1
   UNION ALL 
SELECT c FROM t2

-- 
-- returns:
-- 1    0.229810680294245    
-- 2    0.5750437237982657
-- instead of:
-- 1    0.229810680294245    
-- 2    0.229810680294245    
-- 

The same thing even happens when I do this:

WITH
  t0 AS ( SELECT rand() AS c )

SELECT c FROM t0
   UNION ALL 
SELECT c FROM t0

Any clues why that happens?

Answer 1

Surfacing this reply from Elliott Brossard as a comment to a deleted answer:

RAND() is supposed to run only once when used in a WITH clause. WITH clauses are similar to macros--they are inlined into the query plan--and this is a known bug in that RAND() will be evaluated using different seeds when run on different tasks, but the fix is somewhat complicated.