It appears that the conditions related to 1.16.4 LostCities do not work correctly.

JSON used:

potentialspawn.json
[
{
"dimension": "minecraft:overworld",
"mobs": [
{
"mob": "minecraft:blaze",
"weight": 50,
"groupcountmin": 3,
"groupcountmax": 10
}
]
}
]

spawn.json
[
{
"dimension": "minecraft:overworld",
"mob": "minecraft:blaze",
"incity": true,
"result": "allow"
},
{
"dimension": "minecraft:overworld",
"mob": "minecraft:blaze",
"result": "deny"
}
]

Expected:

Blaze spawn in cities generated in the overworld.

Outcome:

Blaze do not spawn anywhere in the overworld.

The log file did display this message at the beginning of my game session:
[01Jan2021 23:02:53.849] [Worker-Main-14/INFO] [incontrol/]: Enabling support for Lost Cities

With debug mode enabled I saw many messages explaining that blazes were being denied spawning as I flew around chunks in creative mode.

Tested "incity", "instreet" and "inbuilding"

Did you end up finding a solution? I am facing this exact problem, on 1.12.2. Sort of crucial to the project.

@McJty is this functionality working?