question on variable scope and UNION

I recently ran into an edge case and want to clarify my understanding.

The issue is related to the difference between

1. select * { :x :p ?v . FILTER(?v = 1) }

2. select * { :x :p ?v . { FILTER(?v = 1) } }

Where #2 can never return results because ?v is not in scope when the filter is evaluated.

I'm interested in a more complex query that uses UNION. Suppose I have this data:

<http://ex#a1> <http://ex#p1> <http://ex#b1> .
<http://ex#c1> <http://ex#p2> <http://ex#no-match> .
<http://ex#c2> <http://ex#p2> <http://ex#b1> .
<http://ex#a2> <http://ex#p1> <http://ex#b2> .
<http://ex#c1> <http://ex#p3> <http://ex#no-match> .
<http://ex#c2> <http://ex#p3> <http://ex#b2> .

This query will return results

prefix : <http://ex#>
select * {
?a :p1 ?b .
?c :p2 ?d . filter( ?b = ?d )

as will this one

prefix : <http://ex#>
select * {
?a :p1 ?b .
?c :p3 ?d . filter( ?b = ?d )

Now lets combine them with UNION:

prefix : <http://ex#>
select * {
?a :p1 ?b .
{ ?c :p2 ?d . filter( ?b = ?d ) }
union
{ ?c :p3 ?d . filter( ?b = ?d ) }

Whoopts! No results. I believe that the reason is the same as with the simpler FILTER case above: ?join1 is not in scope when the filter is evaluated. Obviously in this case the filters are the same and I can ask the query I want by moving it outside the UNION:

prefix : <http://ex#>
select * {
?a :p1 ?b .
{ ?c :p2 ?d . }
union
{ ?c :p3 ?d . }
filter( ?b = ?d )

But it still seems counter-intuitive to me that adding the union should change the meaning of the query so much.

I'd appreciate any insight on this.

thanks,