Design Issue?

Is there a current known bug with the design? 

Normally, you can only upgrade a part every two days (IIRC). However, it seems that this is now a minimum of 3 or 4 days between parts. My last update to any part was at least 3 days ago now, and when I went through to set the next lot of designs the earliest available date was 9th October (today).

You should be aware that the higher your design % is per part, the more designers it takes to do part upgrades. You'll need to hire additional designers in order to get back on the every-two-days carousel - best practice is to hire enough designers to develop two parts per day (which is around 83 designers).

We're talking 30% parts. 

It's the opposite of what I have experienced in the past. I've had a 90% part before and have been able to throw enough designers at it to have an update available within 48 hours.

Do you just leave the designers on that part or do you rotate them to other parts once a particular update is done?

Rotate them around so that there is always an update the next day, even if it's only one part due to the amount of work required and available designers.