How can I get webpacker's compilation to immediately be reflected in rspec/capybara runs?

I'm running Rails 5.1.4 (with spring), @rails/webpacker 3.4, poltergeist 1.18.1, phantomjs 2.1.1, rspec 3.7.0, capybara 2.17.0.

When I run capybara specs locally, I do not precompile assets, but rather allow webpacker to compile assets automatically in memory. When I have modified assets, I can see in the logs that webpacker is compiling, but when the specs run, the assets are not picked up--in that the javascript does not run causing tests to fail.

When I re-run the same rspec command, there is no compilation, and assets are loaded correctly.