I do not consider this a bug, more of an inconvenience.
When doing squid -k rotate the default value for file
rotations is 10, although the files are small as this is
done after logrotate does its work.
I recommend setting this value to 0 if using logrotate to
rotate squid logs. From  squid.conf "Setting logfile_rotate
to 0 will disable the rotation, but the logfiles are still
closed and re-opened."
BTW After after "upgrading" from 5.2 to 6.0 perfomance of
squid and the whole system was terrible. I noticed the
machine was swapping to much. Without too much time to
investigate i did a clean install of 6.0 and things are ok.