Close to predicted outburst at approx. 9UT 8.December 2023, discovered by Richard Miles.. From BAA Observing campaign news

The cause is probably dust-fountains + liquid, blown out from interior ice and gass pockets. We have such observations from the Rosetta mission.

Or even a possible double nucleus touching when the smaller body is closest to the other, periodically. Close up observations by spacecrafts have shown that comets' cores often are merged by at least 2 bigger bodies. Which may look like the the infinity sign or 8. Also many asteroids comes in pair.

It is NOT Cryovolcanism.

Cryovolcanism has been observed on the moon Enceladus and possible other moons in the solar system.

To make Cryovolcanism, we need 2 things : A heat source in the interior and a liquid above, but under a crust. Foe example the heatsource of Enceladus and Io are tidelforces caused by varying inhomogeneous gravitational fields from Saturn and Jupiter, which stretch and squeeze the moons. Other heat sources can be radioactivity.

We don't have any evidences for that for 29P and 12P.

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

Time: 21:13UT 2023-12-16 Exp. 13 X 65sec
Tel: C14 @ f/7.90 AP7 CCD 1.78"/pix Paramount GT-1100S (Tenagra Norway)
Odd Trondal Obs.code 238 (Uranium). Oslo Norway.

Time: 03:37UT 2023-12-01 Exp. 63 X 65sec
Tel: 0.254m f/7.3 ST-10XME CCD 2X2bin 1.5"/pix
Odd Trondal Obs.code 238 (Uranium). Oslo Norway