View That Unions Two Other Views Does Not Work

I created a view V1 that uses an outer join with a table and calls a sub-view VS1 (ds_proj_report_date) which uses an inner join) and does an inner join with VS1. I also created another view V2 that uses inner join but does not call the sub-view VS1.

When I run the two views as below it works fine

Select * from V1
Union
Select * from V2


I then created another view V3 of the above union as

Create view V3
As
Select * from V1
Union
Select * from V2

Now when I run select * from V3, I get the following error.
Joined tables cannot be specified in a query containing outer join operators. View or function 'ds_proj_report_date' contains joined tables