This is something that comes up quite often, the current design allocation has been this way since July 2018, before this it was much worse!
New Design Balancing system when changing leagues
I feel your pain, in my league I accidentally left after the 4th race (of 20) when leading the championship, I re-joined and my design was reset as per the rules. Last night on race 19 I finally managed to get a driver on the podium again.
I made a suggestion many years ago that a team joining mid season would get the same design as if they were joining in the break (16dp x the number of races in the previous season) This would mean after only a couple of races you wouldn't be too far adrift but as the season progresses you would become less competitive. I think this last bit is important because you wouldn't want an experienced team joining a couple of races before the end of the season with a strong design and potentially interfere with a close title battle between the established managers who had been fighting it out all season.