You used the wrong dynamic action, to store a Grids cell value into a Page Item you have to use FOEX Grid Row/Cell Select Set Item Values.
We have updated your test case on your virtual machine.
The Content Loader you are addressing was setup to "Reload Contents" on APEX refresh -> in your case this should be changed to "Trigger Contents Refresh". We changed this for Btl_Data region, this may also be suitable for the other content loader regions
Important: If you set the value of a hidden item, which should be used by a subsequent action, you have to take care about that the value is set before the subsequent action is executed.
In your case, the refresh of the Content Loader region should be triggered by the On Change event of the hidden item that is used for the drill down (P60_IDBTL_DRILL)
What we did:
Corrected the Dynamic Action on Region Liste
Changed the setting of the Content Loader Region
Moved the refresh of the Content Loader region to the "On Change" Event of the hidden item P60_IDBTL_DRILL