Few things possible, but at 1st verify:
-Both TotalMix and Cubase are ADM enabled (TotalMix->Options->Asio DM).
-If you are recording/monitoring a mono source like guitar, create mono input bus, and use them as input for your mono audio track. if you are recording a mono source using a stereo bus/stereo audio track; maybe that can explain the pan bahavior.
-For better control for monitoring pan and level; use the Control Room instead for the ADM purpose. To do so, well of course CRoom has to be already enabled, so if that not the case you will have to read about the features and how to use the Control Room. if you are already using it, you have to create a Studio channel if no one, and use the Studio Sends tab (available from inspector) to send your monitoring for each audio track that you want ADM. The nice thing here is that you can setup the pan and level as you like, and these settings can be different than the audio track itself thru the mix. So a real must here!
Asus P8Z77-V LE Plus_i7-3770K-3.9GHz_DDR3-16GB_Win8.1-x64_TI FW PCI_1xUAD-2_Cubase 4&7