I should say outright that Flatline pretty much nailed it, and there is a ring in the "normal" M61 dropins- granted it is SIGNIFICANTLY harder to see.
So I'm going to link an IMGUR album with two additional close-ups, adding more images makes my reply way too chunky and I'm not sure that's too "kosher". It's not really needed either....
https://imgur.com/a/bA72anF
M61 219b v2 , ~200 lumens. The "ring" in question is certainly visible on a white wall, but in use it's only visible as a drop in output. The "brighter" outside band of light is a little brighter in real life, making it a little more contrasty to the "dark ring" in question:
M61N XP-G2 ~300lm . We can see where it happens with the regular M61 too, however, it s MUCH harder to notice even on a white wall. It's hardly noticeable in these pics. It takes quite a bit of visual "convincing" to see any sort of ring, I would say even on a white wall it's only visible as a drop in output.
All shots at 5000K white balance, via Samsung S8 RAW, developed in LR mobile. Exposure data not relevant, as I massively tweaked end exposure and values to recreate a the beams to be more accurate to what you see. I copied and pasted settings for each pair of photos. You can see the 100 lm difference in hotspot. Lights 3' from ceiling. What I guess we are seeing is simply the end of the beam and the reflection on the outside from the brass housing. The ring is slightly more noticable IRL with the 219b version, but once you start using it it takes a very flat surface and a keen eye to even pickup the distinct drop in output- however there is no ring (or arc) to make out. Hope this helps. All in all I still love the beam, and ultimately almost all reflectors do this, particularly with shiny rims. Even my MCR-20 and 18s, Reylight Pineapple I can find some sort of "ring" towards the outside of the beam.