LP:732124 - union + limit returns wrong result

create table t1 (a int);
insert into t1 values (10),(10),(10),(2),(3),(4),(5),(6),(7),(8),(9),(1),(10);
select a from t1 where false UNION select a from t1 limit 8;

->
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6 rows in set

Which is wrong as there should be 8 rows in the result

This only happens with union distinct without order by.