6 frequencies of your flights were established 11 game days ago, and 4 freq 5 game days ago. Current game day 17 Sept 90. That means, at time of last month change (beginning of 1 Sept 90), 10 slots were vacant. This is supported by your DOP according to your op. dept. statement. Hence, the warning is valid, and not a bug.
The check is only performed at game month change. As long as you have all your gates utilized at month change, you would avoid it.
