This issue was present on a Samsung Galaxy Tab S6, and an iPhone XR. 

I have googled and apparently this is a bug that has been around for 6 years. 

When you place a Sign block, the Android keyboard that you have installed, will open and allow you to type your message. When you press enter it will go one line down. When you finish typing, there is no way to commit the text to the sign. If you tap outside the keyboard, the keyboard will close and the text will flash onto the sign for one second then dissappear. If you write your message on the sign and press the Android back button, the same thing will happen. There is no way to commit text to the block. For such a normal thing in the game to do, it is surprising this issue has been around for years. 

This issue was observed in Realms and in a normal off-line game. This bug is still unfixed as of 18th May 2020.