Ray not detecting terrain when inside?

I want to detect if a part can spawn depending on its proximity to terrain using Rays, although when the ray is casted inside the terrain, the ray does not detect it. I made this small script to make sure that's what was happening and it pretty much seems like it

local mainpart = script.Parent
local TreeFolder = game.Workspace.Trees
while true do
	wait()
	local right = Vector3.new(mainpart.Size.X/2, 0, 0)
	local front = Vector3.new(0,0,mainpart.Size.Z/2)
	local up = Vector3.new(0, mainpart.Size.Y/2 - .2, 0)
	local directions = {"left", "right", "front", "back"}
	-- right
	local CanPlace = true
	for i = 1,-1, -2 do
		if CanPlace then
			-- right/left side
			for j = -1,1,1 do
				local ray = Ray.new(mainpart.Position + (i*right) + (j*front) - up, mainpart.CFrame.rightVector * 20)
				local hitpart, pos = workspace:FindPartOnRay(ray, TreeFolder, false, false)
				if hitpart ~= nil then
					CanPlace = false
					break
				end
			end
			-- front/back side
			for j = -1,1,1 do
				local ray = Ray.new(mainpart.Position + (i*front) + (j*right) - up, mainpart.CFrame.rightVector * 20)
				local hitpart, pos = workspace:FindPartOnRay(ray, TreeFolder, false, false)
				if hitpart ~= nil then
					CanPlace = false
					break
				end
			end
		else
			break
		end
	end
	print(CanPlace)
end

My question is, is there any way of detecting roblox terrain or a part even if the ray is casted inside of it?