I had the same thing happen looked at maxwells stats using a mod and it seems like he, and all the other cats, have a ripeness status. All the other cats have it set to 3600 and presumably maxwell does too but he also has a spoilage rate of around 2 while all the other cats have that set to zero. Seems like just a bug with maxwell unless theres a reason for him to begin rotting. You can set the the spoilage rate on him using the mod if you really feel like it.