Managed to reproduce the following scenario on my workspace in 4.2:
1. Create new app and export it (app_id=53896 on apex.oracle.com)
2. Create translation mapping, app_id=539001, but don's seed/publish
3. import back the exported app, choose app_id: 539001
The import is allowing to select the same app_id as the mapped translation, and afterwards, the newly imported application is not visible in the builder
(does not appear in the apex_applications view, but it's visible of course in the underlying table: wwv_flows).
Manually chancing the URL parameters in the builder (FB_FLOW_ID,F4000_P1_FLOW:539001,539001), I can see my imported application.
If on point 2, the translated application was already seeded and published, the builder is asking to replace that application, the import succeed,
but the imported application cannot be seen in the builder again. Is that the intended behavior?
I've found a similar problem with apps imported into Apex 4.2 from 4.0 that may be related.
The app appears fine and can be edited but application cannot be run directly from the Application Builder screen. The URL generated has the alias in rather than the actual app id.
An imported app has the URL as below (hover over the Run link from Application Builder)
If you click to run the app you then get the message Page cannot be found. A newly created app correctly has the URL as below:
So it looks like the Application Builder is not picking up the app id for imported apps but retains the &APP_ALIAS_: variable instead. If you edit the app you can run it from within the editing window, just not from the App Builder homepage.