Hello, Merciless.

Thank you for contacting us.  We're aware of this issue and intend to fix it, although it may involve significant re-engineering because, as you know, it occurs infrequently and the steps are usually not reproducible.

Until we fix it, the workaround is to close and re-open the document when this happens.  Don't worry because the underlying data tree is OK.  The problem is only in the "view".