Rob This is related to a bug in iOS 12, for which we have no workaround yet :-( What essentially happens is that the app stores internal configuration data used to secure your connection inside iOS' KeyChain. The KeyChain stores it securely on the device in such a way that nobody can touch it. When the app later needs to make a connection to bunq, it asks iOS for this configuration data and uses it to secure the connection.
Unfortunately, since iOS 12, iOS sometimes returns the wrong data to the app. The connection is no longer secured, so you start to see connection error and the app re-registers itself at bunq and stores new configuration data. After a while tho, iOS 12 then discards the new configuration data and returns the old data to the app: making it even more confusing to the app.
From the app point-of-view, it seems that somebody is messing with the connection, while what actually happens is that iOS is giving us corrupted information :-( We don't have a workaround for this currently: it's very hard to detect it happened and possible workarounds will also compromise the secure connection.
A workaround when this happens is to remove both the bunq and slice app: this forces iOS to clear the stored data from the device.