I have Minecraft on Windows 10, on the Switch, on an iPad and on the Xbox One. On all of these systems except the nintendo switch, both Microsoft accounts can see the other player under "joinable cross-platform friends.".

On the Switch it just says "Your friends are not playing Minecraft right now."

I've tried multiple hardware resets, I've checked privacy settings on both Microsoft's website and the Xbox website, as well as restrictions on Nintendo's website. Multiplayer worlds are enabled on all platforms.

If an invite is sent to the Switch from the PC it receive a notification and can join the game, however it will still not show any available games under the "Friends" tab.