As advised by Unity support I have filed a bug report and hereby open a corresponding thread as requested.
Link to the bug report 622151
Link to additional information (Mixamo forum)...
[Bug Report 622151] Mixamo animation with humanoid rig --> char has an open mouth
Link to the bug report 622151
Link to additional information (Mixamo forum)...
[Bug Report 622151] Mixamo animation with humanoid rig --> char has an open mouth