I believe this has to do with this bug :https://github.com/erlang/otp/issues/5401 which has been fixed with https://github.com/erlang/otp/pull/5409 and was part of the erts 12.1.4 (I believe released with Erlang 24.1.5).
I could not reproducer using your repo :
$ iex --version
Erlang/OTP 24 [erts-12.1.5] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] [dtrace]
IEx 1.12.3 (compiled with Erlang/OTP 24)
$mix test
.....
Finished in 4.2 seconds (0.00s async, 4.2s sync)
1 doctest, 4 tests, 0 failures
