Definitely concerning.
What does the outside look like? Wondering if it oxidation started from within or without...
To those saying tins shouldnt be expected to last more than a couple years... I kinda really disagree with that. Canned foods generally have best by dates as long, effective dates a decade or more. There's 30+ year old camping food in good tins and even older surviving tins of tobacco from nearly every manufacture you can think of...
What's more, I'd say C&D is undisputedly the single biggest proponent and promoter of the "cellaring" process and actively advertise their blends as being designed for such, so when this happens doing just that it shouldn't be blamed on the consumer (unless, again, kept in objectively bad conditions)