The reason is that the default Sales Register uses Collections where only required methods including Inventory Entries Collection has been fetched. Hence, to include Batch Allocations or any other Sub Object, the same is required to be fetched as shown in the example below:
For example,
[#Form: Columnar Day Book]
Local : Collection : Default : Fetch : InventoryEntries.BatchAllocations.*
With the above Fetch, the Sub Object Batch Allocations and the Methods within would be available.