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).