[MC-186153] generation-settings with a FLAT type world doesn't work on servers

Hello! i'm Misterboy64, a player that enjoys playing with others and tries his best to make its private servers good

I've seen this problem basically happened since 1.13...when the type of world that it would've been choosen didn't made work the game at all.

When i put the whole string of the generation-settings command in the server.proprieties in a FLAT world type, the whole server refuses to load that generation-settings
the rest works for me

The type of model of the superflat world that i'm trying to put in is the Tunneler's dream and when i load any word, input, anything next to that command, it doesn't want to work at all, but the moment it is set on Blank, works just fine as water...

Edit: sorry if i kept you uninformed, here's the whole server.proprieties text file

#Minecraft server properties
#Fri May 29 17:31:45 CEST 2020
spawn-protection=16
max-tick-time=60000
query.port=25565
generator-settings=minecraft\:bedrock,230*minecraft\:stone,5*minecraft\:dirt,minecraft\:grass_block;minecraft\:mountains
sync-chunk-writes=true
force-gamemode=false
allow-nether=true
enforce-whitelist=false
gamemode=creative
broadcast-console-to-ops=true
enable-query=false
player-idle-timeout=0
difficulty=hard
spawn-monsters=true
broadcast-rcon-to-ops=true
op-permission-level=4
pvp=true
entity-broadcast-range-percentage=100
snooper-enabled=true
level-type=flat
hardcore=false
enable-status=true
enable-command-block=true
max-players=5
network-compression-threshold=256
resource-pack-sha1=
max-world-size=29999984
function-permission-level=2
rcon.port=25575
server-port=25565
server-ip=
spawn-npcs=true
allow-flight=true
level-name=world
view-distance=10
resource-pack=
spawn-animals=true
white-list=false
rcon.password=
generate-structures=true
online-mode=true
max-build-height=256
level-seed=
prevent-proxy-connections=false
use-native-transport=true
enable-jmx-monitoring=false
motd=A Minecraft Server hoping to be fully updated
enable-rcon=false

And this is the server's log when i run the server.jar

[17:54:11] [main/ERROR]: Unknown biome, defaulting to plains
[17:54:11] [main/ERROR]: Unknown biome, defaulting to plains
[17:54:12] [main/FATAL]: Failed to start the minecraft servercom.google.gson.JsonParseException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1 path $ 
at adn.a(SourceFile:495) ~[server.jar:?] 
at adn.a(SourceFile:545) ~[server.jar:?] 
at adn.a(SourceFile:541) ~[server.jar:?] 
at adn.a(SourceFile:549) ~[server.jar:?] 
at cig.a(SourceFile:187) ~[server.jar:?] 
at ya.<init>(SourceFile:69) ~[server.jar:?] 
at ya.a(SourceFile:73) ~[server.jar:?] 
at yb.<init>(SourceFile:12) ~[server.jar:?] 
at net.minecraft.server.Main.main(SourceFile:81) [server.jar:?]
Caused by: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1 path $ at com.google.gson.stream.JsonReader.syntaxError(JsonReader.java:1559) ~[server.jar:?] 
at com.google.gson.stream.JsonReader.checkLenient(JsonReader.java:1401) ~[server.jar:?] 
at com.google.gson.stream.JsonReader.doPeek(JsonReader.java:593) ~[server.jar:?] 
at com.google.gson.stream.JsonReader.peek(JsonReader.java:425) ~[server.jar:?] 
at com.google.gson.internal.bind.TypeAdapters$29.read(TypeAdapters.java:716) ~[server.jar:?] 
at com.google.gson.internal.bind.TypeAdapters$29.read(TypeAdapters.java:714) ~[server.jar:?] 
at com.google.gson.internal.bind.TypeAdapters$35$1.read(TypeAdapters.java:910) ~[server.jar:?] 
at adn.a(SourceFile:493) ~[server.jar:?]
 ... 8 more

I just changed the type of biome:

mountains --> extreme_hills

....but still that doesn't edit the issue anyhow.