Aircraft config 30 seats but it's somehow blocking itself at 21 seats.
No. That figure, like it says there, is the rolling 7-day average of the route's history. Since you just changed to a larger aircraft it takes week for the average stats to update.
Daily stats are visible below, clicking from "» Click to view expanded statistics"
Also for the slots. For example on route L3 129 (#271455) the slots were created on 1987-11-05, and the route hasn't flown for the first time until 1987-11-28. So 23 days idle is enough to trigger the warning already. If the route is now being flown the warning can be ignored.