I've experienced this as well and have always thought that it may have something to do with the band.
Most cigars burn best for me in the 65% range for me so that is where I keep them. Many B&M's and internet sellers keep them at a higher RH however. I'm not sure if this is correct but my theory is that as the cigar acclimates to the lower humidity of my humidor (dries out slightly) the area that is under the band stays moister because the band keeps it from drying out as fast. Additionally there may also be some extra glue where the band is that may contribute as well.
Not sure if one or both of these things causes it but I have always suspected that the band has something to do with this phenomenon since it seems to happen right about the place where the band was placed.
Interesting. I was attributing it to the composition of the filler. I read somewhere that a type of filler (escaping me) is added to help keep the burn consistent and as it nears the end it is not as prevalent since it would tend to burn hotter, closer to the end. I am rethinking this based on your observation.
I keep my sticks between 62 and 65 rH depending on the time of year (summer vs winter). I smoke them carefully so as not to burn them hot, but that last third just kicks out. Not a big deal as I relight and enjoy the last bit of heaven, but curious none the less.
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