I have noticed this too on my AC68R. DLNA discovery over the network has gotten flaky compared to the latest .49_4 build I was running on before. It is not as bad as it was in the early days of the RT68R firmware in the early part of last year. But it has definitely regressed in the .50 build.
The only change in 378.50 related to minidlna was a fix related to the addition of new files to the database. Minidlna's broadcast itself hasn't changed.