See this https://github.com/8devices/u-boot/issues/3
There is a certain point in OpenWRT history where they decided that kernel command line should never take arguments from a bootloader, period, no matter what. However, the option still lingers in the menu, but doesn't effectively change anything. You can try to play with other menu options but I don't recommend it and it can break your build config easily.
If you need to change bootargs you need to make a build yourself, where you would add a patch or adjust the default command line on your copy of the source code. Unfortunately this isn't really explained anywhere else as far as I know...other than commit messages and bug report pages...