When trying to seal/can Jam/Preserves using the PotRecipe, it will return an empty jar if the item is not in the correct slot or if the first slot is not filled.
Tested with each slot, different kinds of preserver and different amount of preserves.

Results of my tests:
When the first slot wasn't filled - it would return the jam amount (4, 3, 2, 1) - 1, that one would result in an empty jar
When the first two slots weren't filled - it would return the jam amount (3, 2, 1) - 2, that would be empty jar

It voids the amount of slots not filled starting from the first pot input slot.

Input:

Output:

Input:

Output:

The modpack contains the following mods:

1. TFC 4.0.8 Beta
2. Patchouli
3. Ferrite Core
4. Sodium
5. ModernFix
6. Jade
7. JEI