FWIW, I have the full version of JamUp Pro (all in-app purchases). I’ve been using it as IAA in Cubasis with no issues, so maybe this is some odd conflict with another installed app. Make sure to double-click/swipe to clear any other running apps. Sometimes you have to open/reclose an app to truly get rid of it (I have this problem with TC VoiceRack Fx, tho this is kind of an old app)
If the problem persists, try doing a factory reset of your iPad and sparingly re-installing only the apps you really need.
Two different iPads, both with latest applicable iOS installed, and latest Cubasis.
iPad 4
iPad 5thGen(2017)