Turns out that bug is actually due to an undefined difficulty, I guess

ec2.yml

@@ -94,7 +94,7 @@
         terraria_motd: "New major release edition"
         terraria_worldname: "The Home Base"
         terraria_port: 7777
-        terraria_difficulty: 1
+        terraria_difficulty: 0
     - role: terraria
       vars:
         terraria_name: "1401-farm-normal1"
@@ -103,7 +103,7 @@
         terraria_motd: "New major release edition"
         terraria_worldname: "Farman - Normal 1"
         terraria_port: 7778
-        terraria_difficulty: 1
+        terraria_difficulty: 0
     - role: terraria
       vars:
         terraria_name: "1401-farm-master1"

roles/terraria/defaults/main.yml

@@ -1,4 +1,4 @@
-terraria_difficulty: 1
+terraria_difficulty: 0
 terraria_motd: "Please don't cut the purple trees!"
 terraria_port: 7777
 terraria_root: "/var/terraria"

roles/terraria/templates/config

@@ -4,7 +4,7 @@ maxplayers=8
 world={{ terraria_root }}/{{ terraria_name }}/worlds/world.wld
 port={{ terraria_port }}
 password={{ terraria_password }}
-motd={{ terraria_worldname }}: {{ terraria_motd }}
+motd={{ terraria_worldname }} {{ terraria_motd }}
 worldpath={{ terraria_root }}/{{ terraria_name }}/worlds
 autocreate={{ terraria_worldsize }}
 difficulty={{ terraria_difficulty }}