(Didn’t see that in my searches, because of wrong search terms…)
In this thread, a Steinberg rep actually got involved, in the end also assuming it might be a timing issue:
there is another thread in which someone had solve the problem by doing a reset of Totalmix:
This didn’t work for me. What worked was actually using a different USB port. I tried two other ports where I couldn’t either reproduce the behaviour or the occurrence was extremely rare. Going back to the old port immediately immediately let the issue return.