I don't know if it's a known bug, but I deal with this every time I turn off the plugin. This is a default look with no additional plugins or morphs.
Whenever I turn the plugin off, the glutes go to default at first, then squeeze themselves like in the third picture.
I wouldn't actually mind it, but it hides her anus and I have problems making the penis enter into the anus.
I checked the morphs tab, but its clean after turning it off. Must have been to do with colliders or something, that would also explain why it's harder to enter there.