Logrotate - String of junk at top of log files

Logrotate - String of junk at top of log files

I've recently set up the following logrotate job:
/var/log/app.*.log {
daily
rotate 7
copytruncate
nocompress
dateext
}
I've found that when I go to view my log files, they start with some
binary junk. I have to grep using --text and loading the files in vim
takes forever.
Is this expected? Is there anything I can do to prevent it?
Note: logs used to be utf8 text files.