Effects of negative Metaballs do not get evaluated in
ALT-A preview if the Metaball is a child of an animated
object.
It DOES get evaluated in Animation render, though.

Test: Create a Meta plane. Create an UVsphere. Create a
negative Metaball in the UVsphere to make the UVsphere
deform the meta plane, make the UVsphere parent of the
Metaball. Animate the UVsphere diving into the metaplane.

The Metaplane will not be influenced by the Metaball in
ALT-A.