FastAPI rejecting POST request from javascript code but not from a 3rd party request application (insomnia)

When I use insomnia to send a post request I get a 200 code and everything works just fine, but when I send a fetch request through javascript, I get a 405 ‘method not allowed error’, even though I’ve allowed post requests from the server side.
(Server side code uses python).

Server side code

from pydantic import BaseModel
from typing import Optional
from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

origins = ["*"]
app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["POST", "GET"],
    allow_headers=["*"],
)

class data_format(BaseModel):                                                           
    comment_id : int
    username : str
    comment_body : Optional[str] = None

@app.post('/post/submit_post')
async def sumbit_post(somename_3: data_format):
    comment_id = somename_3.comment_id
    username = somename_3.username
    comment_body = somename_3.comment_body
    # add_table_data(comment_id, username, comment_body)                //Unrelated code
    return {
        'Response': 'Submission received',
        'Data' : somename_3
    }

JS code

var payload = {
    "comment_id" : 4,
    "username" : "yo mum",
    "comment_body": "go to sleep"
};
fetch("/post/submit_post",
{
    method: "POST",
    body: JSON.stringify(payload),

    headers: {
        'Content-Type': 'application/json'
    }
})
.then(function(res){ return res.json(); })
.then(function(data){ alert( JSON.stringify( data ) ) })

The error

What should I do to get around this error?
Thanks in advance.