hello, i've seen this question a lot before and i finally found an answer. when you do
/set worldspawnpoint it makes you spawn within 30 blocks from the desired point. if you want to spawn in a specific spot you need a repeating command block. you can get this by doing /give [username] minecraft:repeating_command_block. then place it and turn it to always active. then type /testfor @a. now press done then put a comparator facing away from it then put a regular commandblock in front of it. you can get this by doing
/give [username] minecraft:command_block. then type /tp @a [x] [y] [z]. to test if it works log off the world then go back on. you should be at the exact coordinates you put in the second command block. i hope this helped you.

EDIT: ive seen this in the comments 2 times already so if you want to make it so you spawn in creative/adventure/spectator then add another commandblock next to the commandblock with the tp command in it. then type in /gamemode [gamemode]

the easiest way i found in the comments was by Gealx3 he said turn /gamerule spawnRadius 0 which i find the easiest way to do this.