If you use EOSPlus this means you are using Epic Online Services which provides Crossplay functionality. The Steam OSS should only be used if the game is sold on the Steam Storefront while EOS is used without the need for EOS plus when sold on Epic Games Store.
EOSPlus enables you to mirror functionality from Steam OSS onto EOS OSS allowing the Steamworks SDK to run in parallel with the EOS SDK when using the Steam Storefront.
If you are using a feature that is only available through SteamWorks then EOSPlus probably won’t be able to mirror it onto EOS.