29P's behaviour is interesting, but not so unique because it's distance from the sun ( 6.2 a.u.) is well within where lots of comets already are active. Even much smaller ones. For example : Comet Hale–Bopp of a similar size ( 60 km ), was very active at a distance of 7 a.u. when it was discovered visually at Mag.10.5

This image :
Sun distance (au): 6,16 Perihel 28.March 2019 Sun dist.(au): 5,77

Time: 02:44UT 2023-12-23 Exp. 53 X 65sec
Tel: 0.254m f/7.3 ST-10XME CCD 2x2bin 1.5"/pix
Odd Trondal Obs.code 238 (Uranium). Oslo Norway.