Staying within the strict Robotech franchise and canon all Zentreadi are consistently rendered. However, staying within the strict Macross franchise or combining the Robotech and Macross franchises, the Zentreadi do have varying character designs.
In a 1998 interview, story creator Shoji Kawamori stated none of the Macross material is the real “in-universe” universe. He stated SDF Macross is an episodic “in-universe” animated television show dramatizing actual unseen “in-universe” historical events created for unseen “in-universe” audiences. All the Macross franchise series are entertainment media based on historical events made for unseen “in-universe” audiences. We are “out-of-universe” audiences with access to these dramatized movies and TV shows. We are watching reboots and series taking artistic license.
This is their loophole for canon and continuity.