Custom template pools cannot be loaded by jigsaw blocks

This bug, or part of it, was marked as fixed for version 1.6.2pre1, but it still affects the current version.

Structure pool JSON files in a datapack at data/<namespace>/worldgen/template_pool are reported as Empty or none existent pool in the console when attempting to generate them using a Jigsaw block.

This only seems to occur when the template pool is not included in the natural generation of a structure in a biome file.

To reproduce: load the attached datapack and attempt to load the pool namespace:custom_structure/rooms using a Jigsaw.

It would be very helpful if we could have an example of a datapack which uses this feature correctly (can generate custom structures from a jigsaw block without including them in a biome for natural generation).