I've never seen anyone bring this up before.
Why did Bulstrode seem to gain a new happy face after his only episode appearance. Look at his season 4 face photo:
4 faces. Sad, surprised, mad and really mad.
And then his season 6 face sheet:
Just two faces, the old surprised face and a new happy face. Where did his angry faces go? Why did they sculpt a new happy face for him?
My theory is that in either season 5 or season 6 they planned to bring back Bulstrode for a sort of redemption story. Sort of like Bulgy Rides Again from season 7. Maybe aspects of this unmade episode were reused for Bulgy Rides Again.
Of course there could be a simpler explanation. Maybe the happy face was made in season 4 but wasn't used and didn't get included in the face photos. Maybe it was only made and used for merchandising, for example his 2013 Wooden Railway model.
We may never know.
I honestly feel like Bulstrode got mistreated in the model series. From having an unused nameboard to being decapitated and having his face used on trucks.
#JusticeforBulstrode
As a bonus, here is my attempt at making my own unused Bulstrode faces.