Using outer scope required modules inside an ngx.timer.at handler

Hello,
I've encountered an error which I understand the source to but unsure what's the best practice here.
I have the following example module respond.lua:

local ngx = require 'ngx'
local config = require 'config'

local required_module = require 'module_with_function'

local function handler(_, arg)
   required_module.fn(arg)
end

local function main()
    ngx.timer.at(0, handler, 'test')

    ngx.status = config.code
    ngx.say(config.response)
    ngx.exit(config.code)
end

main()

This would result in attempt to index upvalue 'fn' error thrown from the handler function.
I deduced this is happening because the ngx.exit is called before the call to fn but this context sharing is unintuitive.
If I add a simple sleep before the call to ngx.exit it works just fine meaning the handler function does share the same context as the encapsulating module.

I resorted to adding another require inside the handler function but I would appreciate if there was a clear-cut answer on whether or not modules defined outside the ngx.timer.at handler function are available in it.

For clarification, lua_package_path is set up correctly.

OpenResty: version 1.25.3.1-2

Reproduce the behavior from the respond.lua example using OpenResty 1.25.3.1-2, focusing on the ngx.timer.at handler and the outer required module. Read the relevant timer and module-loading behavior, then clarify whether the example is expected and document the supported practice or the required change.