Gastroschisis- A birth defect where a hole in the abdominal (belly) wall beside the belly button allows the intestines to extend outside of the body. The hole can be small or large and sometimes other organs, such as the stomach and liver, can be found outside of the body as well.
(I think this would be a really interesting lethal Mutation and from personal experience of having a newborn kitten with this condition (R.I.P Hope) they would pass after 1 rollover, As their intestines would slowly start to dry up and the cub would weaken and start to go cold, quickly resulting in the cubs passing)

" 'name' Was cleaning her newly born cubs when she noticed one had their intestines fully outside their body right along side the umbilical cord, the cub seemed perfectly fine otherwise but 'name' knew that this cub wouldn't last"

"As the intestines slowly dried up 'cubs name' quickly started to weaken and eventually went cold indicating that they had passed"

I'm not exactly sure how lineart would look like but I imagine it to be similar to the Harlequin lethal Mutation!

At this time I haven't seen anyone else suggest anything like this but if someone already suggested this pls let me know!