Took me 3 times to set a new start date for the last 2 tasks I've edited. It seems like it's doing this for everyone. Wtf!
It just made no changes the first two times. So annoying...
Thanks for the feedback. It’s not a normal situation. Please try ‘Delete local cache files’ and restart the app. One other thing you can do manually is re-connecting Reminders database on device. (Turn off GoodTask, Reminders and then turn Reminders off on accounts inside System Settings. Turn it back on and check Reminders app refetching the data and then use GoodTask). I need some more investigation on my side to check what’s causing it on your side. I’ll let you know when I need anything. Thanks!
Guys, you tell me to Delete Local Cache files for every single issue I encounter. I've probably done it 8 times now, and it's only worked once.
This seems to be an interface problem, not something relating to the underlying Reminders integration. Otherwise, it'd do it every time. I'll do it agian, but jeez.
Sorry about it. Since it’s not a common issue, it’s hard to replicate. I’ll look into it deeply to prevent it happening. Thanks!
Sure thing 
It seems like just an annoyance but that's only because I visually noticed the date didn't charge. Most people might just assume it did what they told it and then end up missing the appointment.