The release notes of R2019a explain:
First argument to ismethod must be an object
Behavior change in future release
The ismethod function is documented to look for a method of the object that is specified as the first input. However, the ismethod function treats string and char inputs as a class name and looks for the specified method in that class. Therefore, you cannot use ismethod to find a method of an input object that is a string or char array. In future releases, ismethod will return true only if the second input is the name of a method of the first input object. ismethod will not treat the first input as a class name.
For code that uses ismethod with a class name specified as a string scalar or character vector, you can substitute this expression as an alternative that will work in current and future versions.
Then my idea of a workaround:
is not compatible with older and future Matlab releases, but this is needed:
What a pity. If MathWorks has decided, that the first input of ismethod must be a string or char vector containing the class name, my idea would be a nice and clean workaround. Because this is a "change in a future release" we don't know, when the bahavior is modified and using ismethod remains fragile.