Exactly the same issue here. I left my computer running a full scan last night and when I came back the pop up was still up (approx 1hr after scan finished according to the information in scan logs) and it said 100% and the yellow bar was fully across, but there was no writing on it so say that no threats were detected, even though according to the scan log there were no threats detected. This has only started happening since the upgrade to 1488. It used to be that the scan would write “no threats detected” at the end of the scan on the banner thing one it had reached 100%, and then after a few secs it would go away. Now it doesn’t tell you the result and the banner stays up until you physically close it.
I didn’t get a screen shot but the banner is the same as the ones in the images in the OP’s post, only for me it was a full scan not a quick scan. I haven’t tried a quick scan yet, that’s scheduled for this evening.
I’m not sure if it announced “scan has been completed” via audio as it normally would, because I was out of the room at the time it would have finished. Obviously a bug in 1488…