With the release of iOS 12, Apple appears to have placed a 128 character limit on text replacement. At first, I thought this was a bug but it appears to be intentional. After some searching, I discovered a simple workaround using the Google Keyboard.
This is not ideal, but unless Apple releases an update, this is the best option. Once you install the Gboard, you should be able to use your text replacement shortcuts as before.
Hope this helps!