I can reproduce this in Gboard, though not with Samsung Keyboard or SwiftKey. I can also reproduce it in Froala, CKEditor and TinyMCE, which makes me think this is very much a Gboard bug. Unfortunately, that means that there's very likely nothing we can do about it. You should still be able to move the cursor manually at least (obviously not ideal).
I will leave this open for now to see if anything comes up.
Not posted in bugs yet, but, what would cause this? On an android phone when replying in the editor, type a word, and hit enter. The cursor does not jump to the next line it sticks there and every time you press enter the lines break underneath it. So the editor grows vertically. You cannot...
Also Gboard, but only XenForo forums not other forums or social etc.
The cursor actually stays in place but the new line is added. You will see the editor expand if you press enter multiple times. Then click into a line and you will see it is there and typeable and the cursor will move there. You can then backspace out the lines and start typing on line 2. This is the only way I can type on line 2. Press enter and then tap there onto the line as the cursor does not follow it.