Since the submission of this 'bug', I've figured out a different way to get a drowned chicken jockey. So far, I've only tested the following for naturally spawned chicken jockeys, not for baby drowned that will attempt to mount an available chicken.
Jockey separation The trick/observation here is that a drowned chicken jockey will not separate when it spawns inside a water-source block that has air bubbles in it. Either soul-sand (rising) or magma (falling) induced air bubbles will work, although in the last case the chicken will eventually die due to fire damage.
Getting and keeping a drowned jockey To farm/get a drowned chicken jockey, I've made a 10x50x10 water column (all source blocks!) encased in glass, and soul sand on the bottom. After about 30 mins, a first drowned chicken jockey rose to the surface. I've named both the chicken and baby drowned and transported them in a boat. Especially on servers, chicken jockeys may separate randomly; so if you want to keep them: put them in a boat at their final resting place!
Further testing I have yet to test whether the 'hop on' mechanic of a baby drowned will work when the baby drowned is spawned in a water-source block (in stead of flowing water, the situation of the original bug filing)
Since the submission of this 'bug', I've figured out a different way to get a drowned chicken jockey. So far, I've only tested the following for naturally spawned chicken jockeys, not for baby drowned that will attempt to mount an available chicken.
Jockey separation
The trick/observation here is that a drowned chicken jockey will not separate when it spawns inside a water-source block that has air bubbles in it. Either soul-sand (rising) or magma (falling) induced air bubbles will work, although in the last case the chicken will eventually die due to fire damage.
Getting and keeping a drowned jockey
To farm/get a drowned chicken jockey, I've made a 10x50x10 water column (all source blocks!) encased in glass, and soul sand on the bottom. After about 30 mins, a first drowned chicken jockey rose to the surface. I've named both the chicken and baby drowned and transported them in a boat. Especially on servers, chicken jockeys may separate randomly; so if you want to keep them: put them in a boat at their final resting place!
Further testing
I have yet to test whether the 'hop on' mechanic of a baby drowned will work when the baby drowned is spawned in a water-source block (in stead of flowing water, the situation of the original bug filing)