I have also been having this issue [0.16.22]. If two exactly half-compressed belts are merged into a priority output, it combines them into one fully compressed with no overflow, as expected. But if there is more than half-compression on each input belt, the priority output is not filled and some it...

There are plenty of train logic features that would be great. Read train contents, if(train at station), when(train empty), when(train full), etc.