Same shared snapshot dependency being rebuilt sometimes

I have 3 teamcity build config triggered by 3 nightly trigger with 3 different configurations (test/development/shipping), in a build chain structure like the following:

All the compile, package and triggers are being created with kotlin script buildType class, with all the compile configs using the same compile class, packages using the same class, etc,. The difference among them is the configurations (test/development/shipping). And the builds share a same dependency Run UHT. 

The 3 nightly triggers are get triggered at the exact same time. Most of the time, Run UHT only get to run once, which is expected. However, once in a while, 2 Run UHT, of the same CL, same param values, are being built. and when I look into the overview of the Run UHT builds, one of them is "part of" 2 of the say test and development config, and then the other one got rebuilt is of shipping config. 

Looking into the list of "suitable" dependency build criteria, these are the ones I am not sure about. Could it be one of them causing the rebuild?

1. It must have no customized parameters, including those set via reverse.dep. parameters - they do have a reverse.dep param, all set to the same value. can it possibly cause a rebuild? 
2. It must have no VCS settings preventing effective revision calculation, see more details. We are using Perforce: Stream or Client connection settings, or label is specified in Label/revision to checkout option - We are not using labels. We are using stream with feature branch support. What kind of stream connection settings can possibly cause a dependency build not suitable?
3. All the dependency builds (the builds the current one depends on) are "suitable" and are appropriately merged. - what does it mean by "appropriately merged"?