In 18a, the Simulink Arduino support package is not using "arduinodigitaloutput_sfcn" mex file. I think there are files from older versions which are on the MATLAB path. Could you please run the below command and check from where the "arduinodigitaloutput_sfcn" mex file is getting loaded.
>> which arduinodigitaloutput_sfcn
Please post the output of the above command for further help.