SProc works in QA,fails bizarrely in ADP

MSSQL 2000 BE - A2000 ADP FE  afaik I'm fully patched

ALTER       procedure dbo.test

AS
Declare @vTemp int
set @vTemp= 1
if @vTemp = null
begin                  
set nocount on
SELECT    *
from tblformulation
where formulationid = 589
end
else
begin
select * from tblformulationingredients
where formulationid = 589
end
go


In QA this simplified sproc snip below returns results as expected (in this case rs from tblformulationingredients, when run from sproc view in access, i get weirdeness..

my result set has the appropriate number of rows for the formulation from tblformulationingredients ...but it has the schema of the tblformulation

confused ? i know I am

say tblFormulation has 5 fields and tblformulationingredients has 3 in a one to many on formulationid

so
select *
from tblformulation
where formulationid = 589

should return

ID  |F1    |F2   | F3   |F4  |
---------------------------------
589|nom |bom|lom |fom |
      
and 589 has 3 ingredients

select * from tblformulationingredients
where formulationid = 589

ID  | L1 |L2
---------------------
589|foo|bar
589|woo|nuu
589|nam|bam



about as basic as it get s no???

when I run it with @vTemp = 1 i should get rs from tblformulationingredients

589|foo|bar
589|woo|nuu
589|nam|bam

what I get is
ID  |F1    |F2   | F3   |F4  |     <- note this is the schema for the OTHER table????
---------------------------------
589 |      |       |        |      |
589 |      |       |        |      |
589 |      |       |        |      |

so it's like it knows that there are 3 records returned but

if I set the value of @vTemp manually

set @vTemp= null
if @vTemp = null  

 it returns the expected 1 row

but any evaluation which invokes the else fails in this bizarre manner

can anyone reproduce this?

I should note that this is BE/FE combination has been running fine for 5ish years and I've never come across this before...

am i missing something obvious??