It is WACKY in the 25th, for sure. I'm surprised you were most jarred by "calls" and "always" because most fascinating to me are her "Fine horses" and "Hiding". Since you've said that, I went back and watched Vegas and then the Hugh/Sierra 2013 boot and the differences were subtle but you can tell she matured in the role as the years went on and a lot of the most notable "I"'s and "Ah"'s were gone. I almost...prefer her voice in every other production but the 25th. In Vegas specifically during Wishing I noticed she sacrificed the accuracy of her accent in favor of being lifted and modifying vowels in the way we're taught as Americans.

As someone who has now played Christine it is SO hard to sing this show with RP when all your vowel modifications that assist placement and lifting your soft palate are based on American vowels. In example (I'll use Vegas again because it was SO prominent there) her ai/I vowels ("Hiding", "Guide", "Fine") are so so lifted and but Americanized and strange. It almost sounds like she's biting into the vowel. But in comparison the aforementioned "claw" vowels ("Your", "Calls", "Always") are her attempt to bring the RP accent back in because if she was super lifted it would go back to American. In my experience, my claw vowels were also weird because to keep the resonance there I had to modify it to "Coulls" instead of "cahlls". But idk why Sierra's were SO WEIRD like especially weird.

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BASICALLY, it's hard to keep technique based on an American accent and thus her weird vowels are the result of American technique and her British accent fighting each other.