I've personally never had issues with it on Arch; I agree with the sibling comment that some issue with groups might be the the issue. In case it helps, here's what I always do to set it up on a new installation:
1. Install `sudo` via pacman
2. Optionally set the EDITOR env var to use something other than vi for the following command, e.g. `export EDITOR=nvim` (if you do change it and want to retain the default shortcuts like `ctrl+a` to go to the beginning of the line, you'll also probably want to do `set -o emacs` to make sure it doesn't get changed due to the EDITOR being set)
3. Run `sudo visudo` (or `sudo -E visudo` if you want to make sure the `EDITOR` env var is used) to edit the sudoers file and look for the line `%wheel ALL=(ALL:ALL) NOPASSWD: ALL`. Uncomment that one, and comment out the one slightly above that's the same but doesn't have `NOPASSWD`. Save and exit the editor.
4. If you're not already a member of the `wheel` group, add yourself with `usermod -aG wheel <your username>`. You might need to log out and back in or reboot to this to take effect; groups are often weird like that.
As an alternative, Arch has `opendoas` in the `extra` repos nowadays, which is a port of the OpenBSD replacement for sudo designed to be easier to configure. There's literally no configuration for it by default, and to enable passwordless, all you need to put in `/etc/opendoas.conf` is `permit nopass :wheel`.
