Can 'circleci tests split' be used per directory?

I’m running rspec with the following settings.

TESTFILES=$(circleci tests glob "spec/**/*_spec.rb" | circleci tests split --split-by=timings)
bundle exec rspec $TESTFILES

This setting causes strange rspec errors, probably because there are too many files.
Therefore, to set by directories.

TESTFILES=$(circleci tests glob "spec/{${DIRNAMES}}/" | circleci tests split --split-by=timings)
bundle exec rspec $TESTFILES

The rspec test passed, but I got the following warning.

“Error autodetecting timing type, falling back to weighting by name. Autodetect no matching filename or classname. If file names are used, double check paths for absolute vs relative.”

Is this setting the problem?