It's not season 18, nor are the others you mentioned just "random other series." Detailed discussion can be found in previous threads, as you know since you've replied to one of them that has links to the others.
Thread 1: viewtopic.php?f=18&t=48690
Thread 2: viewtopic.php?f=18&t=48801
Thread 3: viewtopic.php?f=18&t=49461
For God knows what reason, Channel 5 has started the current series as season 6 (not 18) under the new-to-Channel-5 name Traffic Cops
. If anything (as mentioned elsewhere), there should likely be a new "Traffic Cops (2018)" series created with season 6 as the first available. It can't go back under the original series (previous network and now three series names ago) for various reasons, not least of which because there's already a season 6 there, nor can we arbitrarily call it season 18 because no official source calls it that (especially not Channel 5, the source of truth) and because the intervening series the scene calls 15-17 are already properly accounted for here under their official names and series numbering.
Admins are going to have to decide how best to resolve this.