Permission denied to a 777 file

I have a Django app which is configured to write db queries into a /tmp/db.log file.

        "debug_console_to_file": {
            "level": "DEBUG",
            "filters": ["require_debug_true"],
            "class": "logging.FileHandler",
            "filename": "/tmp/db.log",
        },

The application usually starts from a uwsgi server using www-data as a user. However, sometimes I manually run it (with python manage.py) via my user - tvelichkov.

The problem is that when I try to start it from my user, I got a Permission denied error to /tmp/db.log file since www-data has already created it (removing the file can temporary fix the issue, but then the server will receive the same error, since now my user owns the file).

    PermissionError: [Errno 13] Permission denied: '/tmp/db.log'

I've tried to add my user to www-data group

    $ members www-data
    www-data tvelichkov

I've also tried to give 777 file permissions to the file.

    $ ls -l /tmp/db.log 
    -rwxrwsrwx 1 www-data www-data 22102 юли 30 15:25 /tmp/db.log

But I still keep getting this Permission denied error. Any help would be appreciated.

Note: I used to have no issues with this setup in Ubuntu 18.04, but now I have it in Ubuntu 20.04.

UPDATE: Here are the permissions of the /tmp/ folder, but note that this is after I ran chown root:root /tmp and chmod 777 /tmp as suggested by @adrian vera, BTW after this change seems like chmod 777 /tmp/db.log is working, because I no more get the Permission denied error. Is it possible that Ubuntu changed something for the /tmp/ folder between versions 18.04 and 20.04 ? Because I'm pretty sure I didn't change anything to this folder before running into this issue?

    $ ls -l / | grep tmp
    drwxrwxrwx  24 root root       4096 авг  3 10:19 tmp

Note2: I have one more machine with clean Ubuntu 20.04 install where I have this issue too, I will double check the permissions there too once I got home.

UPDATE2: So on a clean Ubuntu 20.04 install the permissions of the /tmp/ folder are:

    $ ls -l / | grep tmp
    drwxrwxrwt  23 root root      12288 авг  3 16:41 tmp

And this is how the /tmp/db.log look like:

    $ ls -l /tmp/db.log 
    -rw-r--r-- 1 www-data www-data 0 авг  3 16:54 /tmp/db.log

    $ sudo chmod 777 /tmp/db.log 
    $ ls -l /tmp/db.log 
    -rwxrwxrwx 1 www-data www-data 0 авг  3 16:54 /tmp/db.log

    $ lsattr /tmp/db.log 
    --------------e----- /tmp/db.log

    $ whoami
    tvelichkov

    $ getent group www-data
    www-data:x:33:tvelichkov

    $ python manage.py test --settings=cs.settings.test
    Traceback (most recent call last):
      File "/home/tvelichkov/.pyenv/versions/3.6.10/lib/python3.6/logging/config.py", line 565, in configure
        handler = self.configure_handler(handlers[name])
      File "/home/tvelichkov/.pyenv/versions/3.6.10/lib/python3.6/logging/config.py", line 738, in configure_handler
        result = factory(**kwargs)
      File "/home/tvelichkov/.pyenv/versions/3.6.10/lib/python3.6/logging/__init__.py", line 1032, in __init__
        StreamHandler.__init__(self, self._open())
      File "/home/tvelichkov/.pyenv/versions/3.6.10/lib/python3.6/logging/__init__.py", line 1061, in _open
        return open(self.baseFilename, self.mode, encoding=self.encoding)
    PermissionError: [Errno 13] Permission denied: '/tmp/db.log'