Thanks for bringing this to our attention. What you are seeing is new behavior in LayOut that we are trying - as of the 2026.2 release - where you are able to continue working on your document while viewport(s) are being rendered. Previously, the application would prevent most operations while a viewport was being rendered. Interestingly, you should still not be able to modify the viewport that is currently rendering, but it seems like you are doing exactly that.
For now, it would be wise to wait until the render has finished before making further changes to the viewport, which is the workaround that you seem to have already discovered yourself!
Adam