Hello,
I am using 3D Coat 2023.40 - Recreate with:
New Voxel Sculpting Project
Choose Cube
Press Enter to apply cube in scene
Enable Symmetry (X-Axis)
In Sculpt Tree, Rightclick on volume object, select "To Global Space"
in Quick Access menu, choose "Resample" with a value of 2 or 4
Symmetry Plane disappears.
Repeat step 5 -> symmetry plane appears again.
Note that the symmetry plane does not disappear when choosing different values than in step 6.
I hope this helps!