Erik,
Here is the link to the raw files:
https://www.hightail.com/download/ZWJYTGsrdzhrYUJvSWNUQw
When I originally made the switch from a Canon 5D to the IQ180, I felt the way the 180 handled this type of highlight was more natural. As you say, it could be several things. The exif data says it was a 2 sec exposure on the 180 and a 1 sec on the 3100. If anything the 180 image should be clipped more given the longer exposure and the lower DR. But as you know the recorded shutter speed on these backs is not all that accurate when using a copal shutter, so I can't say for sure the 180 image was longer. They were both set to ISO 100 and the same f-stop.
The lighting was difficult on purpose, but identical for both. In fact the lens flare was pretty close to identical in each shot. I used to refer to this purple fringing from saturated areas as "blooming", but I'm not sure that is the right term. I looked at a similar shot from the a7rII on a Batis 25. It handles this better than the 3100 but not as well as the 180. Again many different variables so who knows.
I will repeat this is more of a curiosity than any sort of "issue"; it just isn't that different to worry about. For me, the decision to upgrade will basically be 1) live view and 2) staying on the upgrade discount path vs. the cost of doing so. Very rarely do I want to shift/rise/stitch anything wider than my 60xl, and the 3100 seems to handle that just fine up to 18mm. And unlike Voidshatter I have no interest in very long exposures or in pushing shadows beyond minor adjustments. So (again for me) any upgrade will have nothing to do with image quality.
Dave