Robert Comsol wrote:
One of the biggest advantages of TAS was that it was an animated TV series, thus there wasn't any need anymore to "compromise" as they could
have visualized anything
they wanted without considering budget restraints.
To supplement Christopher's comments above, I am guessing that the life support halo was an "automatic" effect, generated with an optical printer in some fashion. The way the halo sometimes aligns with the characters exactly and other times is offset looks to me like a lens artifact (zooming, focusing).
Even if the halo was not an automatic effect, it would take an artist far less time to trace a quick outline than to draw the detail of a TOS-style environment suit—especially if archival cels were reused, as suggested above.
Does it present a "continuity" error with later TREKs? Sure, but what doesn't? The shuttlecraft in TAS doesn't look like the winged bread box in TOS. (And the spin-off Treks did more time traveling and dimension hopping than the Time Lords of Gallifrey, but that's a subject for a different thread.)