Steps to reproduce:

1. Open attached project "multiline.zip"
2. Open scene "repro"
3. Run it
4. Into the InputField, enter some text (the key point is that text would contain newline symbols (Enter))
5. Select InputField in hierarchy and change it to single line
6. Notice that multiline text is still present

Note: If text is put to new line automatically (because it doesn't fit in the line) then setting InputField to single line will result in correct behaviour, because text didn't contain any newline symbols.