I have now verified that the issue of the Leica S2 + Contax adapter “forgetting” the mounted lens is either an issue of the Leica S2 or the adapter itself.
The problem is triggered 100% reliably when the camera is activating the standby mode.
Regardless, which Contax lens I have mounted (80/2 Planar, 120/4 Macro, 140/2.8 Sonnar, 210/4 Sonnar, extension tubes 13 and 26mm), my Leica S2 will reliably forget that a lens is mounted.
The only remedy to this problem at the moment is to unmount the Contax adapter from the S2 body and remount it.
The Leica S2 will regain all lens information the moment, the adapter is remounted immediately and will work flawlessly as long, as it is prevented to go into standby mode.
This includes even camera off/on switches!
If I prevent the camera from going into standby mode and switch it off and on again, it will still have the correct lens information.
If the camera has been going into standby mode and is then switched off and on again, it will still be inoperable and have no lens information until the adapter is unmounted and re-mounted again.
This is either an issue with my Leica S2 + Contax adapter or a firmware related issue (both camera and my Leica S lenses are running the latest firm ware updates. My Contax adapter reads a later version of the lens firmware than my S lenses!).