Hi everyone, 
We did some investigation and saw that this does keep happening to a number of designers. 
I was not able to re-create this issue yet, so if anyone can help out, do let me know. Some key things to know would be: 
- which models were affected? A link would be helpful
- if anyone can do this on demand (you know which change sets model to free), do let me know
- are there any specs for your operating system? There's a possibility this could be browser-related, so any notes would help me a lot. 
Feel free to contact me directly and we'll try to solve the issue as soon as possible.