We recently (a few months ago) switched from Q3 2017 to R1 2019 SP1, and traced the origin of this exception to switching to the new version. I.e. it did not have this issue with the older version.
We are using implicit styles and it only happens in the 2013 theme, at least among the themes that we use.
If we press Alt, and then "F" to get to the backstage, we get the following exception. After the exception everything works normally, but you have to press Alt again to get the access text to show on the backstage.
Object reference not set to an instance of an object.
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)