The difference is also likely because Kalafina isn't an active group. They performed together for 10 years and Wakana was part of FJ at the time too. It's a lot easier than you think to forget how to sing those songs. Even for me, if I don't sing a Kalafina song in over 3 months because I rotate the music I listen to, there is an adjustment period. They hadn't sung together in 7 years prior to last year, and currently they hold lives together once a year. The rest of the year is purely just solo activities.
Wakana's solo repertoire very rarely demand the use of her head voice, with the only songs that do being Butterfly Dream and Boku no Kokoro no Tokei. She doesn't sing Kalafina songs too often in her live shows because she has quite a large pool of songs in her solo discography, and the Kalafina songs she likes to sing often aren't vocally demanding (Musunde Hiraku, Kimi no Gin no Niwa) or she will skip over the Kajiurago entirely (Oblivious). What she has gotten better at is vocal dynamics and emoting which she doesn't incorporate into her "Kalafina singing" for some reason. Not singing everything in forte would lessen the strain immediately.
Hikaru's solo repertoire literally never exceed A4 because her chest range has severely diminished, and she uses her falsetto even less now. I have a feeling Hikaru has nodules based on the excessive amount of air in her upper chest range, which indicates a lack of proper vocal cord closure, a clear sign of vocal nodules. I don't think she has gotten surgery either from the sounds of it, perhaps due to the risk of making her unable to sing entirely like it did to Julie Andrews.
In contrast, Keiko is the only one who explored and expanded her comfortable range by deliberately going out of her vocal comfort zone of depressed larynx chest voice, and she kept up being a part of FJ after the intial disbandment so she never lost her harmonisation skills. Her most recent solo lived focused solely on her upper range because she wanted to be more comfortable there, and because of that her vocals never really diminished the same way (plus she always had scarily accurate intonation). Her problems are stylistic, not technical, when she gets too excited during energetic songs and especially her "cutesy" voice when she raises her larynx to the roof.
And I'm only saying this because I love Kalafina. Criticism, not hatred, because I want and know they can do better.