FLATTEN shifting fields incorrectly

Follow up from PIG-5201, PIG-5452.  

When flatten-ed tuple has less or more fields than specified, entire fields shift incorrectly. 

Input

A       (a,b,c)
B       (a,b,c)
C       (a,b,c)
Y       (a,b)
Z       (a,b,c,d,e,f)
E

Script

A = load 'input.txt' as (a1:chararray, a2:tuple());
B = FOREACH A GENERATE a1, FLATTEN(a2) as (b1:chararray,b2:chararray,b3:chararray), a1 as a4;
dump B; 

Incorrect results

(A,a,b,c,A)
(B,a,b,c,B)
(C,a,b,c,C)
(Y,a,b,Y,)
(Z,a,b,c,d)
(E,,,,E)

E is correct.  It's fixed as part of PIG-5201, PIG-5452.
Y has shifted a4(Y) to the left incorrectly. 
Should have been (Y,a,b,,Y)
Z has dropped a4(Z) and overwrote the result with content of FLATTEN(a2).
Should have been (Z,a,b,c,Z).