What does it do, what do you expect it to do?

It calls io.input():lines(), I expected it to call io.input():lines(...)

The Lua manual doesn't really mention this use-case, it does say however:

The call io.lines() (with no file name) is equivalent to io.input():lines()

Is the manual wrong / incomplete? Does it actually behave the way you want it to in vanilla Lua?

Well I found the OC bug while porting Lua 5.2's IO lib (the C code) to LuaJIT/FFI

If it behaves different to "vanilla" Lua, feel free to make a pull request to change it, with an example use-case and way to reproduce the discrepancy. Since it's basically undocumented behavior, I don't really think it's critical, though.

Well you could say

The call io.lines() (with no file name) is equivalent to io.input():lines() (Emphasis mine)

Would allow io.lines(nil) to be equivalent to io.input():lines(), thus io.lines(nil, ...) should be equivalent to io.input():lines(...).