Different voice tone, and Mask, and Chest plate for that matter… Only thing worth keeping consistent is the tint of Vader’s eyes. Almost impossible to fix the other stuff…
But why try to “fix” any of it? So his suit changes between films. It doesn’t break continuity for Vader to get upgrades.
Just the red eyes in IV and in Rebels. If you look at at the movies from a trilogy point of view, there is a continuity irritation with some fans. If you just Watched Star Wars IV and never saw the other ones, well then there are no issues.