Setting modifier using xmodmap

I am using GNU/Linux (Devuan) with the mate desktop and with i3wm. I would like to assign mod1 to Meta_L. If I show the current assignment with

$ xmodmap

I get

mod1        Alt_L (0x40),  Meta_L (0xcd)

So I tried the following:

$ xmodmap -e "clear mod1"
$ xmodmap
xmodmap:  up to 4 keys per modifier, (keycodes in parentheses):
...
mod1      

and then

$ xmodmap -e "add mod1 = Meta_L"
$ xmodmap
...
mod1        Alt_L (0x40),  Meta_L (0xcd)

So, I am getting the old assignment back.

I am not sure what is happening. Am I doing something wrong or is xmodmap or some other program ignoring / overriding my assignment?