Assignment statement does not evaluate all its expressions first

[Xmilia Hermit <xmilia.hermit@...>]

As stated in the manual at 
https://www.lua.org/manual/5.4/manual.html#3.3.3 the assignment 
statement should first evaluate all its expressions and only then the 
assignments should be performed.
However, this seems to not always be the case.
The following code shows the problem:

local up = {}
setmetatable(up, {__newindex=function()
    up = ""
end})

local function test()
    up.x, up.y = 1, 1
end

test()

This failes with: attempt to index a string value (upvalue 'up').
However, I would expect from the manual that
a=up, b=up, c=1, d=1 are evaluated (in any order) as they are 
expressions and then the assignments
a.x = c, b.y = d are assigned in any order which would not result in the 
error observed.

Am I missing something from the manual or is this a bug?

Regards,
Xmilia