I am the opinion that this is bug. I am sure that I have seen similar situations in the past, but it had not really bothered since /n works (I remember situations where even /n did not work and I needed to logon again to cancel the old logon), but the main goal is to complete my work instead of being scared to death and wanting to fled from the screen.
I my opinion the messages are there to help me to proceed, which means I correct the entries till I am done and save my work.
I would take OSS note 65470 as evidence that such non-operating buttons are unwanted behavior.
But I would also check if this behavior was not caused by an enthusiastic screen and transaction variant fetishist, see 192911 - F3, F12, F15 not possible in transactions w.variant
