From #10350 (which contains a unit test to demonstrate the issue)

If you have a non-catchable pattern in requirements like f.e. a positive lookahead (.+(?=/foo/)), the generator will not accept the parameter as the parameter itself cannot fulfil the requirement, but only matches in the context of the entire path.

I'm not sure how/if you would like to fix this, this only adds a failing test case. One way might be to generate the URL and run the matcher against the entire URL.

/cc @Tobion