Ruby on Rails test fails or succeeds depending on how the test suite is called

I'm working my way through the Hartl Ruby on Rails Tutorial (4th Edition) and I've come across a problem with some tests I'm trying to run - specifically the exercise in section 11.2.3. All of my tests are green up to this point.

I test the new piece of code:

vagrant@ubuntu-18:/vagrant/sample_app/test$ rails test:mailers
Started with run options --seed 59483

  1/1: [======================================================================] 100% Time: 00:00:00, Time: 00:00:00

Finished in 0.33304s
1 tests, 9 assertions, 0 failures, 0 errors, 0 skips

Everything is groovy.

I attempt to run the full test suite:

vagrant@ubuntu-18:/vagrant/sample_app/test$ rails test
Running via Spring preloader in process 11914
Started with run options --seed 56533

ERROR["test_account_activation", UserMailerTest, 1.2505091589991935]
 test_account_activation#UserMailerTest (1.25s)
ActionView::Template::Error:         ActionView::Template::Error: undefined method `merge' for nil:NilClass
            app/views/user_mailer/account_activation.html.erb:6:in `_app_views_user_mailer_account_activation_html_erb__2910351700178259135_47130470135880'
            app/mailers/user_mailer.rb:10:in `account_activation'
            test/mailers/user_mailer_test.rb:9:in `block in <class:UserMailerTest>'

  48/48: [========================================================================] 100% Time: 00:00:01, Time: 00:00:01

Finished in 1.71036s
48 tests, 206 assertions, 0 failures, 1 errors, 0 skips

Bugger. It's definitely the same piece of code that's failing in one place, and working in the other.

However, if instead of running rails test I run rails test:run, then this happens:

vagrant@ubuntu-18:/vagrant/sample_app/test$ rails test:run
Started with run options --seed 50303

  48/48: [========================================================================] 100% Time: 00:00:01, Time: 00:00:01

Finished in 1.72194s
48 tests, 215 assertions, 0 failures, 0 errors, 0 skips

And similarly, if I use rake test or rake test:run, then everything is fine.

vagrant@ubuntu-18:/vagrant/sample_app/test$ rake test
(in /vagrant/sample_app)
Started with run options --seed 9429

  48/48: [========================================================================] 100% Time: 00:00:01, Time: 00:00:01

Finished in 1.70475s
48 tests, 215 assertions, 0 failures, 0 errors, 0 skips
vagrant@ubuntu-18:/vagrant/sample_app/test$ rake test:run
(in /vagrant/sample_app)
Started with run options --seed 50280

  48/48: [========================================================================] 100% Time: 00:00:01, Time: 00:00:01

Finished in 1.73529s
48 tests, 215 assertions, 0 failures, 0 errors, 0 skips

Can anyone explain what's going on here? And ideally how to make just a rails test call work again?