Got it. The correct command to reproduce the problem locally is:
CI=true RAILS_ENV=test LOAD_PLUGINS=1 SKIP_DB_AND_REDIS=1 RAILS_DB=nonexistent bin/rails runner "puts 'booted successfully'"
All those envvars are important. Couldn’t get it working for RAILS_ENV=development. Without CI=true and LOAD_PLUGINS=1 in test mode it does not appear that all plugin classes are loaded.