I tried it again at Defender high settings, and this time, no problem. Either the file was already whitelisted, or the scanning is behaving inconsistently, or, like you said, the system was just busy with something else.I have no idea. I also configured 'Defender high settings' and even installed OSArmour (default). Everything is normal. But except OSArmor, I do not use another non-Microsoft security.
Please, check it after some minutes, because Windows can slow down when updating Windows Store, performing some scheduled tasks, etc.