Cannot find compatible Agent when using shared resource and enabling parallel tests

We have a Buildconfig running Tests with a Connection string as a shared resource. Lock is readlock on any available value.

When enabling parallel tests the child builds spawned by parallel tests build feature won't start because 

There are no idle compatible agents which can run this build

and the Incompatible agents list shows all agents with

Implicit requirements: teamcity.locks.readLock.DatabaseConnectionString defined in Build step: Integrationstests ausführen

Current batch size is 2. There are 9 entries in the shared resource and no other builds running that use this shared resource.

When disabling parallel tests i can start 9 builds in our buildconfig without a problem.

Any ideas on why this happens? We are on 2023.05.3 (build 129390)