This morning I received a call from someone in McDonald's management. I believe he is in charge of more than one McDonalds. He told me he had investigated my complaint (probably the one I sent them several days ago, before this latest experience).
He said he had determined that the Frappe machine at this location was indeed malfunctioning, and was not giving the amount of Frappe it was designed to give. He said he had worked on the machine for about 2 hours today, and he believes the problem is now corrected. He also told me that language in the employee training materials is confusing, and he will try and get it changed. Some instructions refer to a "fill line", and some to a "cap seat". He said that with Frappes, the cup should be filled to the "cap seat" rather than the "fill line". I understand the "cap seat" is the area at the top of the cup where the plastic changes shape and the lid seats on the cup.
This person who called me offered his apologies, and I think this is a reasonable conclusion to my complaint. This being a holiday (Memorial Day), I was impressed he was even working today.