Ugh, that sounds like a nasty little bug @rakel, I'm sorry that you've hit it!
From what you are describing, it sounds like you have hit a run-time exception. I am not able to reproduce it on my Windows 10 machine, so I'm wondering if you can reproduce it after you completely close out of mockups, and then re-open it.
Let me know if it persists through restarts, and we will dig deeper. Sorry again for any trouble this has caused you!