That caught my eye too… asymmetrical sooty smoke can’t be a good sign.
This looks like a cool white plume from the left side of the mid-section of the booster, becomes sooty when it passes the exhaust plume. In both my screenshot and yours, it is very definitely just on the left-hand side.
My best guess is that methane is venting for some reason (I can’t think of a good reason) on the left side (long arrow), and when it reaches the exhaust plume it gets hot enough to burn, but its sooty because it isn’t mixed properly?