As you can see, a custom addons directory was set using this Steam launch argument:

01:02:20.826 ENGINE       : CLI Params: -addonDownloadDir D:\SteamLibrary2\steamapps\common\Arma Reforger\reforgeraddons -addonsDir D:\SteamLibrary2\steamapps\common\Arma Reforger\reforgeraddons -forceUpdate

However, it still uses the old default directory in addition to it, even though I did not list it in the -addonsDir parameter.

01:02:20.828 ENGINE       : Addon dirs:
01:02:20.828  ENGINE       : dir: 'D:/SteamLibrary2/steamapps/common/Arma Reforger/reforgeraddons'
01:02:20.828  ENGINE       : dir: './addons'
01:02:20.828  ENGINE       : dir: 'C:/Users/Til/Documents/My Games/ArmaReforger/addons'

This can result in major issues when for whatever reason mods are present in the old directory. For example, the Experimental game downloaded mods with the same GUID to the My Games path, which the Stable game then tried to use, resulting in a script compilation error when loading into the game on Stable with any of these mods.
It took me quite a while to figure out the reason for this, the described behavior should absolutely not happen.