Yeesh. Everyone is jumping the gun on this development real hard as if Aka-sensei hasn't been an absolute madman storyteller for over 130 chapters already. I don't remember seeing anyone having real problems with this on comment threads a week ago.
I interpreted this as Kaguya being so overloaded by everything she has no idea how to handle new developments, and so falls back on the mentality that's always carried her through stressful situations in the past, even though it's the side of her that she's actively repressed and hated. She doesn't really want this part of her to come out, and I don't think it's an active decision at all on her part, but she's facing an unprecedented situation and doesn't know what to do.
I did and still do welcome this new development. Bakaguya is a joy, but Icey is honestly just as large a part of who Kaguya is at this point, and if this side of her is never dealt with or addressed...well, it wouldn't be a very good or otherwise healthy relationship.
Is it a regression? Yes, but not in a way that contradicts development. It's already been explicitly stated that Icey was very affected by the Dual Confessions and sincerely loves Miyuki. It's a return to roots, and rather than seeing this as outright personality changes. The one that initially fell in love with Miyuki is having a confrontation with a boy who may or may not know how to handle what's coming.
And that's what this is. A confrontation, no more and no less.