The picture on the left is a fan model for a Battlefront 2 mod by Orthohex. It’s good but not canon and not 100% accurate.
But Chris is correct that Hot Toys made their own interpretation of Rex’s helmet instead of doing an actual realistic Phase 1 and 2 merge.
The visor is a weird shape, but I believe it’s because they used the animated style helmet’s visor as reference instead of a realistic Phase 1 helmet. It matches this promo shot from the show which Hot Toys tried to replicate.
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Which looks especially jarring because the rest of the helmet is mostly realistic. Overall they’ve been doing a somewhat odd part animated and part realistic approach for their Clones.
It’s not a deal breaker and the figures are still fantastic, but there are little details that are off. It’s the little details that many of us Star Wars fans really like to nitpick but I just try not to think about it.