I've not used that app, but I just enabled it as default so that I could see what settings are there.
Did you already go inside the Messages app settings to "Notifications"?
In there, at the bottom is a setting for "Preview messages" that was enabled in mine. This says when enabled, it will show a preview in the status bar and on the lock screen.
Try un-checking that (if you hadn't already) and test to see if it solves your issue.
If that doesn't do the trick, there's a couple other places to experiment with.
Sent from my Verizon Galaxy Note4 using Tapatalk