Unfortunately this has been a bug for a long time. I often get similar problems.
Unfortunately, again, it seems to come up so "randomly" that it's practically impossible to build a workflow that the developers can follow to see the problem for themselves. If they can't find the problem they can't fix it.
In my experience, this and other similar issues seem to happen more often when I've been working on the same document for a long time. Scribus seems to get slower and less responsive as I plough on through the work. Once it gets too bad I close Scribus then re-open the document and carry on until the next time it happens and repeat.
It feels like it might be a problem with the Undo stack filling up memory or something but that's a total guess. (It's the only thing I can think of that should get bigger alongside the problem getting worse.)
However, again, telling the developers to follow a workflow of: "Do loads of random things for a couple of hours and you'll start to see the problem." won't be very helpful to them.
My only suggestion would be to bounce Scribus whenever you go for a break. Not a very nice solution but it works, until the next time at least.