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;

->
+------+
| a |
+------+
| 10 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
+------+
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.