Bug or feature on field display on overloaded lua add() method

I am generating display methods for some fields in lua scripts. The example in the documentation at https://wiki.wireshark.org/LuaAPI/TreeItem:

local t = tree:add( proto_foo, buf() )
t:add_le( proto_foo.fields.u16, buf(1,2), 100, nil, "(", nil, "little", 999, nil, "endian", nil, ")" )

suggests that the display will be:

"Unsigned short: 0x6400 ( little endian )"

When I imitate this code with:

proto.fields.packet_type = ProtoField.new("Type", "type", ftypes.UINT8)
function display_type(value)
  if value == 0 then
    return "Heartbeat"
  end
  if value == 1 then
    return " Start Of Session"
  end
  if value == 2 then
    return "End Of Session"
  end
  if value == 3 then
    return "Application Message"
  end
  return ""
end
function dissect_packet_type(buffer, offset, packet, parent)
local size = 1
local range = buffer(offset, size)
local value = range:le_int()
local display = display_type(value)
parent:add(proto.fields.type, range, value, display)
return offset + size
end

With the display method I see:

"Heartbeat"

I want it to print like the standard fields so instead I generate the display function as:

function display_type(value)
if value == 0 then
return "Type: Heartbeat (0)"

end
if value == 1 then
return "Type: Start Of Session (1)"
end
if value == 2 then
return "Type: End Of Session (2)"
end
if value == 3 then
return "Type: Application Message (3)"
end
return ""
end

Is my function invalid? Does the documentation need to be updated? Or is this a bug?

Thanks in advance.