Cat owners, please help!!

Totally off topic for this app but I don't know where else to ask! I asked the vet and she just brushed it off, says my cats healthy and fine. 
The issue is at night, my cat will not use her litter tray for a poop, only a pee! She will use it easily in the day for both but when it comes to the night, that poop goes on the carpet 😖 I'm baffled! We do have another cat but she's always been fine up until recently! I can't pinpoint any changes either! No different litter or anything.. 
She's a ragdoll (6 years old) just completely baffled as to why she's doing this! 
Thank you for any answers :) 
Yes the litter box is cleaned as soon as a mess is done (very stinky) so I'm still stumped 😂