Seems like Fixed in Build cant be automatically set if the build was started not on default branch.
Also there is one related problem: the monitored branch name can't contain slash character, but this is quite possible situation with gitflow branches. For example SourceTree names gitflow branches with feature/ hotfix/ and release/ prefixes.
Guys, do you have any ideas on how to organize YouTrack-TeamCity integration, i.e. automatic Fixed in Build for builds on all branches from the branch specification?