To recreate the problem:

Edit any table for the hordes to spawn mobs and add any attribute to a mob. Ex:

[
	"minecraft:creeper{Attributes:[{Name:\"generic.movement_speed\", Base:0.6}]}-35-0-0"
]

Now in game use commands to start a horde with the table you modified:

/hordes start @A 5000 hordes:default

No mobs will spawn or the fallback table will be used.
The json file has no problem when validating with https://jsonlint.com/ , so I don't think it is a syntax problem.

For some reason now it works. It was probably a mod conflict from my part.