CONNECT BY fails on some combinations of VIEWS and UNION ALL

The simplest query that reproduces the probem, we found so far:

with
    t0 as (select 'a' a, 'b' b),
    t1 as (select a, b
           from t0
		   CONNECT BY a = PRIOR b)
	SELECT a, b
	from t1
    UNION ALL
	SELECT null, null;

Workaround
Put the query containing the CONNECT BY into a subselect:

with
    t0 as (select 'a' a, 'b' b),
    t1 as (select * from (select a, b
           from t0
		   CONNECT BY a = PRIOR b))
	SELECT a, b
	from t1
    UNION ALL
	SELECT null, null;