Thanks for the question.

I think that this a normal variation among otherwise red garlic. I have grown Chesnok Red garlic for several years and have found that sometimes a resulting bulb has a purple appearance. Generally this coloration is only on the covering around the bulb and not the clove itself. The taste seems unaffected by this. Most garlic contain the purple pigment anthocyanin. This is a water-soluble pigment that can turn blue or purple under acidic conditions. Since soil pH can vary from season to season, this may be why you have not observed this on a regular basis. Additionally this color transformation tends to occur more often with immature garlic. So depending upon when you harvest your garlic, it may not be always be present.  This might also extend to different maturation times among separate garlic plants,  Perhaps this could explain the purple garlic in the picture you sent. With my own garlic I have observed that a purple coloration can differ among cloves even within the same head of garlic. I do not think that this is anything you need to be concerned about. Vietnamese Red is an excellent garlic for cooking. I would guess, however, that in past harvests you often saw purple stripes on various bulbs between individual cloves,

Thanks for bringing your question to our attention.