When a Tracking Dimension is setup for serial number tracking, and set as 'Active in sales process', then the 'Blank issue allowed' is checked on automatically and can't be turned off.

Because of this, when using the packing screen, it is possible to use the back arrow and the item is accepted without serial number.

When setting up Tracking Dimension for Serial Number and set it up as 'Active' generally, it is possible to check 'Blank issue allowed' off. Make it possible for the 'Active in in Sales Process' as well.