newrelic-ruby-agent: Upgrading from V8 to V9 causes transactions with custom names to disappear in UI and errors in the logs

I’m in the process of upgrading a large Rails 6.2 app running APM 8.16 to version 9.0.

In the transactions interface on some web transactions (seems to be the ones with custom transaction names) I see now that most time is attributed to a segment by the name #. What is this and why?

In the newrelic.log I can see the following:

[2023-03-06 15:13:18 +0100 web-01 (1107039)] ERROR : Exception during Transaction#finish
[2023-03-06 15:13:18 +0100 web-01 (1107039)] ERROR : NoMethodError: undefined method `name' for nil:NilClass
[2023-03-06 15:13:18 +0100 web-01 (1107039)] DEBUG : Debugging backtrace:
/home/app/a/shared/bundle/ruby/2.7.0/gems/newrelic_rpm-9.0.0/lib/new_relic/agent/transaction/abstract_segment.rb:267:in `force_finish'
/home/app/a/shared/bundle/ruby/2.7.0/gems/newrelic_rpm-9.0.0/lib/new_relic/agent/transaction/abstract_segment.rb:91:in `finalize'
/home/app/a/shared/bundle/ruby/2.7.0/gems/newrelic_rpm-9.0.0/lib/new_relic/agent/transaction/tracing.rb:52:in `block in finalize_segments'
/home/app/a/shared/bundle/ruby/2.7.0/gems/newrelic_rpm-9.0.0/lib/new_relic/agent/transaction/tracing.rb:52:in `each'
/home/app/a/shared/bundle/ruby/2.7.0/gems/newrelic_rpm-9.0.0/lib/new_relic/agent/transaction/tracing.rb:52:in `finalize_segments'
/home/app/a/shared/bundle/ruby/2.7.0/gems/newrelic_rpm-9.0.0/lib/new_relic/agent/transaction.rb:555:in `commit!'
/home/app/a/shared/bundle/ruby/2.7.0/gems/newrelic_rpm-9.0.0/lib/new_relic/agent/transaction.rb:534:in `finish'
/home/app/a/shared/bundle/ruby/2.7.0/gems/newrelic_rpm-9.0.0/lib/new_relic/agent/instrumentation/middleware_tracing.rb:111:in `call'
/home/app/a/shared/bundle/ruby/2.7.0/gems/unicorn-5.8.0/lib/unicorn/http_server.rb:634:in `process_client'
/home/app/a/shared/bundle/ruby/2.7.0/gems/unicorn-5.8.0/lib/unicorn/http_server.rb:732:in `worker_loop'
/home/app/a/shared/bundle/ruby/2.7.0/gems/unicorn-5.8.0/lib/unicorn/http_server.rb:548:in `spawn_missing_workers'
/home/app/a/shared/bundle/ruby/2.7.0/gems/unicorn-5.8.0/lib/unicorn/http_server.rb:144:in `start'
/home/app/a/shared/bundle/ruby/2.7.0/gems/unicorn-5.8.0/bin/unicorn:128:in `<top (required)>'
/home/app/a/shared/bundle/ruby/2.7.0/bin/unicorn:23:in `load'
/home/app/a/shared/bundle/ruby/2.7.0/bin/unicorn:23:in `<top (required)>'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:63:in `load'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:63:in `kernel_load'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:28:in `run'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:481:in `exec'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:31:in `dispatch'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:25:in `start'
/home/app/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.25/exe/bundle:49:in `block in <top (required)>'
/home/app/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/friendly_errors.rb:128:in `with_friendly_errors'
/home/app/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.25/exe/bundle:37:in `<top (required)>'
/home/app/.rbenv/versions/2.7.2/bin/bundle:23:in `load'
/home/app/.rbenv/versions/2.7.2/bin/bundle:23:in `<main>'

The custom naming in rails controller looks like this:

NewRelic::Agent.set_transaction_name("#{NewRelic::Agent.get_transaction_name} - search_backend: #{resolver.backend}")

I can see here that it seems to be parent which is nil https://github.com/newrelic/newrelic-ruby-agent/blob/68a34050cff2034559ea1acc59132d3ee245ce42/lib/new_relic/agent/transaction/abstract_segment.rb#L267 - How can I further debug this and what might be causing it and could it be related?