Actually there are two problems:

1. Time of calculation
QB2 checks for repository changes when a build is triggered (build condition). It later uses this revision for the checkout in the build. This works fine, if the build starts to run immediately. In our environment the build often has to wait in a queue and when it finally starts, the revision is outdated.
-> check again for svn changes when the build starts or make this behavior configurable

2. Aggregate builds:
If there are several builds in the queue and they get aggregated, the properties from the first one will be used. For the SVN revision this means that the "oldest" one gets executed, not the newest.
-> With aggregation use the latest build.