Bundler 2 introduced various incompatibilites und confusing behavior. To add to the confusion, Bundler's behavior changed after the release of their version 2.

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

Prevents you from data breaches and liability risks
Upgrade at your own pace
Works with modern Rubies

A Migration Path to Bundler 2+

Rails Long Term Support