Whenever you add a .mcpack addon to a world with custom structures, they will only generate in new chunks until you save and go back to the title screen. If you go back into the world, and load more new chunks, any custom structures won't generate. I flew around through new chunks for about 10 minutes and my structures, which spawn in all biomes except oceans and rivers, are nowhere to be seen. However if I take the addon off the world, reload it without the addon, go back to the title screen, add the addon back, and then load it again, then fly through more new chunks, they will start generating again. 

For some reason, custom structures will only generate the first time you load the world after enabling the pack.

 

I'll paste the feature_rule.json text here as well in case there is some problem with it causing the issue

{"format_version":"1.13.0",
"minecraft:feature_rules":{
"description":

{ "identifier":"brdl:talloutpost_feature_rule", "places_feature":"brdl:talloutpost_structure" }

,
"conditions":{
"placement_pass":"surface_pass",
"minecraft:biome_filter":[
{"all_of":[

{ "test":"has_biome_tag", "operator":"!=", "value":"lakes" }

,

{ "test":"has_biome_tag", "operator":"!=", "value":"ocean" }

,

{ "test":"has_biome_tag", "operator":"!=", "value":"river" }

,

{ "test":"has_biome_tag", "operator":"!=", "value":"nether" }

,

{ "test":"has_biome_tag", "operator":"!=", "value":"the_end" }

,

{ "test":"has_biome_tag", "operator":"!=", "value":"beach" }

,
{
"any_of":[

{ "test":"has_biome_tag", "operator":"==", "value":"overworld"}

,

{"test":"has_biome_tag", "operator":"==", "value":"overworld_generation" }

]
}
]
}
]
},
"distribution":

{ "iterations":1, "scatter_chance":0.05, "x":0, "y":"query.heightmap(variable.worldx,variable.worldz)+-7", "z":0 }

}
}