Should an SCN obtained from one RAC node be immediately usable on other RAC nodes?

I'm seeing ORA-08181: specified number is not a valid system change number and don't understand why. The error happens around once a month from a process that runs every few minutes - so very rarely. Running oracle 2 node RAC (11g-r2) linux. Scenario:

on instance 1:

1. get an scn (my_scn) via dbms_flashback.get_system_change_number


2. use my_scn in some queries with no problems: select ... from X as of scn my_scn


3. pass my_scn to a parallel pipelined function that is used in a query (Note the time from obtaining the scn in step 1 to passing the scn in step 3 is less than a minute and potentially less than a second.)

on instance 2 (oracle chose the instance based on parallel query) in the parallel pipelined function:

1. use my_scn parameter in a query like select ... from Y as of scn my_scn


2. very rarely get ORA-08181: specified number is not a valid system change number

So, my question is: Should an SCN obtained from one RAC node be immediately usable on other RAC nodes?