This is a weird-edge case for Windows. Some functions can return a string path with Window's separators (i.e. vim.fn.stdpath('data') -> C:\path\to\localappdata). If I were to try and join them like so:

local Path = require("plenary.path")
local data = Path:new(vim.fn.stdpath("data")):joinpath("dir/file.lua")

the resulting data will be a mixed path like C\local\app\data/dir/file.lua.

The proposed solution changes this so that the result is C:\local\app\data\dir\file.lua, and therefore keeps the behavior in line with other modules such as pathlib from python.

NOTE

For the additional test to pass for both Windows and Linux, the additional :gsub("\\", sep) was added.
This means that:

local Path = require("plenary.path")
local data = Path:new("local/app/data"):joinpath("dir\\file.lua")

would result in local/app/data/dir/file.lua even in Linux.

Although I fail to see where this could be an issue, it can be removed.