Welcome to the forum and the wonderful world of sous vide.
Kind of difficult to work out what the ‘issue’ might be without having a few more details, not the least of which would be a description of what you consider ‘odd.’ Did the brisket get any pre-cooking treatment? How thick was it? It is thickness and not weight that determines how long it takes for heat to penetrate your product.
36 hours is really not very long for cooking brisket sous vide. Brisket is a tough piece of muscle and at 57.5C, a relatively low temperature, the conversion of collagen to tender gelatin is slow.