I think this problem exists since 6.21.
It still remains in 6.3.

When the logs are being reloaded, mIRC will first try the last logfile available and read it, starting from the bottom, until it hits the first empty line. Then it will continue with the next logfile.

What makes the difference in re-producing this seems to be the "Lock log files" option. Disabling that will resolve the issue.

Reload Logs wasn't working for me at all until I disabled "Lock log files". Works now though, thanks

In fact, i don't think todays logfile is loaded at all. I just had the impression it was because when i join the channel, the last messages obviously also appear in the logile. As they have just been written there.

I guess mIRC first opens the logfile (with OF_SHARE_DENY_READ maybe) and cannot open it a second time for reading back the buffer.

Same problem
when mirc reconnects,
it does not load the contents in the channels.

Thanks this has been fixed for the next version.