Yesterday upgraded on Pi3 from 18.04 to 19.04 with Software Updater and so far so good.
When expect image with 19.04?
I do not expect there to be a 19.04 Pi image.
How did you change it to Ubuntu MATE 19.04? When I did my Ubuntu MATE, the latest version for Pi was 18.04.
In the the settings of software updater, you can change it from looking for future LTS Releases to all versions