TechnoTypeClass::LoadFromINI reads the object's info from a given rules-ish file (rules, gamemode, map). If the ini file contains the [object] section, and the object is Voxel=yes, it calls ObjectTypeClass::Load3DArt, which unloads the voxels associated with this object and reloads them. Naturally, the reloaded data will exist in a different memory location. (Even if the files are the same as they were in the last file, let's reload them anyway! Loading times? Pshaw.)
VoxelAnimTypeClass::LoadFromINI takes the voxel from ShareSource and saves a pointer to it into this Voxel Anim. But if the Voxel Anim's section is not in the current rules-ish file, this function won't run - the Voxel Anim will keep a pointer to the old voxel file content, which possibly doesn't exist anymore.
##### STEPS TO REPRODUCE #####
rulesmd.ini:[SomeVoxelAnim]
ShareBodyData=yes
ShareSource=MTNK
gamemode ini/map code:[MTNK]
Something=Something
##### ADDITIONAL INFORMATION #####
All maps in the singleplayer campaigns already redefine MTNK, LTNK and HTNK to have BuildTimeMultiplier=1.0. Hence this bug is practically guaranteed when VoxelAnims are ShareSource'ing those units.
Insanity is fun.