This happens on any multiline text input components, in Qt Quick, Controls 1 and 2, like TextArea or TextEdit.
Using the SwiftKey keyboard on Android, if the cursor is the very start of a new line, pressing backspace will not delete the line. The cursor will stay at the start of the line and not move 1 line up. Nothing is deleted. Effectively the only way to remove a line break would be to use "selectByMouse: true" to select and cut the line break, which is not a valid workaround at all.