It really doesn't make sense to use YY here, and I'd argue it's better for it not to work than to allow people to use the wrong year code and then not realise until the next new year when they start seeing dates with the wrong year on them.
YY is the ISO year, which ties in to the ISO week. It will not give you the year you expect in day-month-year dates close to a new year, and you should never use it in them.
More discussion here: Viewer Pane, Image properties are okay?
Maybe the bug is that it works anywhere. It should probably be blocked from working in the custom date formats at all, and reserved for things like rename presets.