It could also be that this Mass is not an official Shrine Mass in the sense the it was organized by the shrine. I believe that any priest with proper facilities can get permission to say a “private” Mass in a shrine chapel (say for a visiting group or somehting similar) without it being a Mass organized and promoted by the Shrine itself. That would explain why it wasn’t on the shrine schedule or that the volunteers at the desk didn’t have complete details about it.
Let us try to be charitable in our reactions, expecially toward those who are giving their time to staff our wonderful shrine. I wish I was available mid-day to attend a Mass at the shrine (latin or otherwise). We do usually make it there every few months for a special event or just to soak in the beauty.