The developers have written the following (see reddit) to prevent the bug:
We're seeing many of these reports appear to be specifically related to starting a multiplayer session from the Expeditions Outrider camp. If you're concerned that this issue may happen to you we recommend avoiding this setup for the time being as this greatly reduces the likelihood of the issue occurring.
That is, don't start your multiplayer sessions from the camp but from Trench, for example. Also, your teammates can join the session even if you have already chosen a campaign. Meaning, start an expedition of your choice and when you are inside, ready to run, your friends will join first (join your game). This should prevent the bug for the most part.
However, the developer gives another tip:
A potential prevention method to safeguard yourself if you run into this issue > The moment you notice that your gear has disappeared, immediately force close the game, then reboot it.
So have your ALT + F4 or SHIFT + CTRL + ESC ready and kill the game before the lost inventory can be saved from autosave. This way you should still be able to do something about it in case of emergency and avert the error.
The error should also mainly occur during expeditions, which means that no one will stop you from boxing your way through the campaign together with your buddy until level 30.
The important thing is this:
The bug doesn't occur as often since the last patch as it did before, but it hasn't been fixed yet. The developers have been working hard since 04/11/20221 to find and fix the bug. If you are affected by the bug, you will get your items back but probably not all of them. The recovery of the lost items will happen as follows:
So you will probably get back the most expensive and valuable items you lost, but the rest will remain gone.
Outriders Inventory bug fixed since 04/15/2021:
People Can Fly and Square Enix have released a statement about it via Reddit:
We have just released a patch on Steam, Epic, Xbox and PlayStation platforms that addresses the bug that has, since the previous update, been causing the inventories of certain players to be wiped.
If your inventory has been impacted, please note that our efforts to restore your items are a separate process from today's released patch. Restoring the inventory of affected players is still our highest priority, while today's patch is intended to prevent the issue from continuing to occur. The restoration process is run via our server database and does not require a patch to be implemented. We will have news of this very soon.
While we are confident that the patch will prevent inventory wipes in future, we do want to use the weekend to continue monitoring the situation and ensure that no more players are encountering any related issues before we move onto the restoration process.
A small patch (version 1.06) finally fixed the inventory bug on 04/15/2021. Unfortunately, other bugs have resulted from it.
New issues in multiplayer games:
With this new patch, some players may occasionally encounter multiplayer connectivity issues followed by inventory items being momentarily invisible. This is a known and only temporary behavior that is part of our three-step inventory safeguarding measures.
Restarting your game or waiting a few minutes and then restarting your game should make your items appear again.
The fact that connection issues can occur is already known to the developers, however there are other issues: