That effect is common in Microbolometers with a BST sensor, however what's going on in the Therm-App is completely different (The therm-app doesn't use a BST sensor).
That artifact is a result of using the stock Therm-App app in "Night Vision" mode. This mode works to use image processing to exaggerate temperature differences for a more detailed image at night. Sometimes the image enhancement goes a bit overboard and causes the halo. However I think this is on purpose and can be useful as if you had, say, a warm person on a background with only slightly lower temperature, it would be very hard to see the person. However, with the image processing, the slightly warmer person will have a black halo around it which makes them much easier to detect.
Personally I use the "ThermAppPlus" app on the Play store, which has other image processing enhancements for lower noise (as I generally use the Therm-App for thermography), however I guess the night vision mode in the stock app results in increased detail.
I'll post a comparison soon.