Border Collie Puppy – Toilet Training regression

Looking for some advice regarding regression on toilet training for my 4 month Border Collie puppy.

When i first got her, she came from a licensed breeder from a farm (an actual farm, not a puppy farm just for clarification). So i knew she had never been in a house and toilet training would take time.

First two weeks there were a lot of accidents. Next two weeks were much better and accidents were significantly down and often could be attributed to my mistakes and bad timing. She did her business in the garden but would rarely do anything on a walk (Expected as she is excited and focusing on all the new smells)

However this last week she has seemed to regress. She still pees outside but poos inside. No matter what i do or how often she is walked out let out into the garden, she poos inside (Luckily often in the bathroom but still not great)

She will be out in the garden, pee have a smell then walk straight back in the house and poo.

I'm not sure how this has happened. I'm going back a few steps to get her back to pooing outside but I'm concerned as what has caused this and if it's something I've done or can avoid doing in future.

Any help would be appreciated