Nion unstable while ringing out AES patchbay

Nion #1- NIO 8ml II, AES, AES, NION 8o
Nion #2- AES x4
Nions connected via XDAB

All AES inputs/outputs are normalled to a patch bay.  When using a 1K tone generator to patch into the signal path from the Nion, the AES cards output digital noise and lose their normal at the patch bay when the signal generator is pulled from the patch bay.  The nion must then be restarted to restore the normalled path in the patch bay.  Is this normal behavior? 

I have done the digital noise workaround that Ivor provided that mutes the AES channels that don't have audio on them and it still exhibits the same issue.  Is there a reason why patching into the path of an AES signal would cause the nion to be unable to re-obtain a lock and not exhibit this problem?  Thanks and your responses are greatly appreciated.