[6.10.3] Logrotate 3.18.1 causing errors

Since upgrading from 6.9 to 6.10.3, I'm getting errors for a logrotation:

keyword 'size' not properly separated, found 0x3d

My configuration for my job is:

/var/log/foobar/debug.log {
  su root root
  size=1M
  rotate 5
  notifempty
  missingok
}

Looking into this, the problem is with logrotation 3.18.1 which is in Unraid 6.10.3:

https://www.linuxquestions.org/questions/showthread.php?p=6253137