Kevin
No, if you leave and re-join the same league mid-season you get the design reset as per the thread I posted a link to (above). The reason I know this is I've been there, done that, got the T shirt!
I mis-clicked and accidentally left PiR League in May last year (Season 11) after the 4th race and suffered for the remainder of the season.
The other league members may also need to watch out because next season the OP will have a design which is higher than the remainder of the league because of the N+1 bug. To overcome this he will need to reset his car again.
Ah ok, design points returned correlate to the races run before leaving? But wait what’s the N+ 1 bug??