Heroku deploy Ruby 2.6.1 : warn_for_outdated_bundler_version - ruby-on-rails

Question

This question is related to this one, but the proposed solutons didn't work for me.
I'm trying to deploy a Rails 5.2 application with Ruby 2.6.1, but I keep getting this error in the deployment phase:
/app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/lockfile_parser.rb:108:in `warn_for_outdated_bundler_version': You must use Bundler 2 or greater with this lockfile. (Bundler::LockfileError)
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/lockfile_parser.rb:95:in `initialize'
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/definition.rb:83:in `new'
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/definition.rb:83:in `initialize'
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/dsl.rb:234:in `new'
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/dsl.rb:234:in `to_definition'
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/dsl.rb:13:in `evaluate'
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/definition.rb:34:in `build'
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler.rb:135:in `definition'
from /app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler.rb:101:in `setup'
from /app/vendor/ruby-2.6.1/lib/ruby/2.6.0/bundler/setup.rb:20:in `<top (required)>'
from /app/vendor/ruby-2.6.1/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
from /app/vendor/ruby-2.6.1/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
from /app/config/boot.rb:3:in `<top (required)>'
from /app/bin/rake:2:in `require_relative'
from /app/bin/rake:2:in `<main>'
What I find strange in this error is this line:
/app/vendor/bundle/ruby/2.6.0/gems/bundler-2.0.1/lib/bundler/lockfile_parser.rb:108:in `warn_for_outdated_bundler_version': You must use Bundler 2 or greater with this lockfile. (Bundler::LockfileError)
Heroku seems to say it is using Ruby 2.6.0. But in the build phase, it says it uses Ruby 2.6.1 with Bundler 2:
-----> Ruby app detected
-----> Compiling Ruby/Rails
-----> Using Ruby version: ruby-2.6.1
-----> Installing dependencies using bundler 2.0.1
The build is successful, but the deployment is in failure.
Here is how is configured my app.
Gemfile (reduced to what matters)
ruby "2.6.1"
gem 'rails', '~> 5.2'
Gemfile.lock (reduced to what matters)
RUBY VERSION
ruby 2.6.1p33
BUNDLED WITH
2.0.1
On heroku, my buildpack is heroku/ruby. My stack is heroku-18.
Heroku seems to be supporting Bundler 2, as said in this post.
Any idea on what could fail in the deployment?

Answer 1

I found a solution. The problem was at deploy time, not during the build. The deployment was using Ruby 2.6.0, because of Heroku's web dyno that is by default:
rails server -p $PORT -e $RAILS_ENV
Yet, this post from bundler github is telling to prefix every single command with bundle exec. But the rails server command from the dyno is not prefixed so.
A workaround I found is modifying the project Procfile file, and add the following line:
web: bundle exec rails server -p $PORT -e $RAILS_ENV
This way, the rails server command uses the expected Ruby version, the 2.6.1 insteand of the 2.6.0 that generated the error.