Any E6 kit lacking a Formaldehyde step will cause images to be less stable.
Kodak and Fuji include it in the kit in the Bleach Pre Bath. It can also be in the Stabilizer or Final Rinse. ALL current final rinse baths do NOT contain Formaldehyde. Therefore the current C41 and E6 Final Rinse baths will not protect your film properly.
Make sure that any final rinse for E6 films contains Formaldehyde if the Pre bath is missing.
This warning is not true for C41 films made after about 2000.