I can confirm this behavior on a Galaxy S9 too .. I hadn't noticed this before, but like all above, I agree, a timeout is essential for a mobile platform that depends on battery. I feel that Evernote is making an end-run around the battery saving features of the Android platform by refusing to time out.
If EN want to make it easier for a certain population of users not to have to take any action to keep the screen going, just add this as a setting. I think there is a large number of users who prefer to save on battery life.
Thanks.