Found when running LiipFunctionalTestBundle's test suite again symfony 3.4.
The issue is that phpunit's @runInSeparateProcess feature is very risky as it requires all files required before, even-though no all file are require-able as such.

Some ideas to fix this:

1. enable processIsolation=true in the bundle's phpunit.xml.dist, which is free from the issue
2. not split the container in several files for tests
3. find a way around in phpunit-bridge

I hope 3 is actually possible, that'd be best.