This is “unsolved” because it isn’t a bug/issue, and this behavior is not weird. I’m sure it has been frustrating, and appears to be an issue/bug from your angle, but multiline textboxes do not commit on hitting “enter” on the keyboard as that is how you drop to a new line. This is intended functionality for a very broad spectrum of multiline textboxes, not just in UE4. In filling out your post here in the discussions, for example, you hit enter multiple times. If it had posted the second you hit enter (instead of adding a newline), would that have been satisfactory? Probably not. Keep this in mind with your game’s UX.
To modify the behavior, you can easily handle the pressing of enter as a commit, and the pressing of shift + enter as adding a new line. Basically, if shift + enter is pressed, we do nothing (thus allowing regular functionality – a new line – to carry out). If enter is pressed, we handle it as a commit. See here:
This example is rudimentary, and in your position I’d also want to make absolutely sure the multiline textbox is focused as well (in case you’d ever want enter to do something else, like traverse a menu, save, etc.). However, it does show an example of working through exactly what you’re attempting, and gives a great starting point.