greasepencil can only add one lattice at the moment, if more can be used later.
although lattices can be used to add lattices, this results in the need to add many different greasepencil
This is resolved with GPencil: Remove limitation to use only one Lattice modifier.
See https://wiki.blender.org/wiki/Reference/Release_Notes/2.93/Grease_Pencil for the 2.93 changes.