It happend, because when we drive from Portland under Staunton in tunnel, game still is on Portland Island (and game don't load Stauton Island collision - all vehicles fall to blue hell and respawn in tunnel).

Workaround for it (tested)
<code lua>
Get_Player_Island = 0

bindEventHandler("OnResourceReady", thisResource, function(event, resource)

setInterval(function() -- timer for function second
    OneSecond()
end, 1000)

end)

function OneSecond()

if (localPlayer) then

    local getPlayerIsland = gta.getIslandFromPosition(localPlayer.position) -- workaround for https://bugs.gtaconnected.com/view.php?id=403

    if (localPlayer.position.z < 0.0) then
        if (getPlayerIsland ~= Get_Player_Island) then
            if (getPlayerIsland ~= 0) then

                gta.setIsland(tonumber(getPlayerIsland), true, true, true)

            end
        end
    end

    Get_Player_Island = getPlayerIsland

end

end
</code>