Dissolution Version 0.3.13r2
Dynamic Stealth Version 1.12.2.120
Fantastic Lib Version 1.12.2.068

Issue:
With only these 3 mods installed possessing an undead is impossible as you are booted out of the body of the undead. Though for whatever reason when you are booted out of the body you are still tethered to the undead creating an issue where you cannot move at all with a shaking screen or having your soul self be stuck with the undead attached to you despite not possessing it.

How To Replicate:
Install Dissolution mod from 1.12.2 section of a mod now called requiem on Curseforge
Install Dynamic Stealth and Fantastic Lib
Load into the world in creative mode, open the chat box, and click the text talking about refusing to leave this world
Spawn a Husk and us ethe /kill command to die
Right click on the husk to possess it and see the issues stated begin

Additional Notes:
For a good frame of reference as to how the mod is meant to work I recommend just using it alone and possessing a mob.