I have noticed several times, and once more right now, that sometimes Rumble displays this alert box, while I did not do anything with the "welcome block".
Now for instance I just shift+suppr to delete the 4th block that was a duplicate of 5th block.
Moreover it did not only display the alert box once. It popped it repeatedly in a high-frequence fashion (it flashed something like 5-7 times in a second before becoming quiet).