PD hasn't fixed this yet. I'm baffled.
Also because it happens not only on PS4 (my case) but also on the PS5, which is supposed to have a complete new type of architecture and very fast loading/saving times.
I searched for this thread (not the first one I'm seeing about this specific issue) because this started happening again yesterday on my end.
Imagine if it took me an entire day to make the livery and I tried to save it when I was done with it, only to realize it couldn't get saved and got rendered endlessly.
Luckily, it gets stuck only now that I want to add a couple of stickers and a shadow effect. The whole livery is still pretty decent and can be loaded. But no matter what I do (even changing that specific sticker and save it) it doesn't render it. The circle keeps spinning for hours, so all I can do is quit GT7, re-enter, and see that nothing new has been saved on it. Obviously.
EDIT: I'll try
@StPeterMartyr 's workaround when I have time. Thanks.
EDIT 2: Nope this method didn't work for me.
But I previously made a copy of the same design because I knew it was giving me problems. So today I deleted the most recent one, loaded the "old" one (identical in decals and identical in issues), made the few changes on it, and oddly enough it did work. It saved the design. Then I managed to save the style and upload it, and closing GT Auto I had no problem...