Hello! So I looked into this for you and what I found was kind of interesting.
So it appears that this is actually working correctly, but it is a bit confusing, so let me explain.
The InitialPlayerMode only is used for players who have never been assigned a mode in the past. Modes are driven by oxide permission groups, so if a player already belongs to the group "warmodepvp" then the InitialPlayerMode will not apply to them - it will only apply to people who have never joined your server since WarMode has been installed.
If you want to make sure everyone has a "clean slate" then first unload WarMode, then run "oxide.usergroup removeall warmodepvp" this will of course make it so no one is in the pvp group. Then, if you have InitialPlayerMode set to pve, when you reload the plugin, everyone should be pve now as they do not already belong to another mode group. Keep in mind this is assuming you haven't added any other custom modes - if you have, you'll just want to clean those up as well.