1 - Wooden stick in vicinity listing ( or inventory as discovered after tesing)
2 - Clicked and dragged it onto pile of 5 rags i had in backpack, selected "craft into splint" (or whatever the words are)
3 - Red message appeared in bottom left - "Rag is ruined" (or very similar)
4 - Hovered over stack of 5 rags in backpack and it said "ruined" in the info box
5 - i split the stack up incase it was just reading the top rag and there might be a usable one underneath but new stack were also listed as ruined
6 - i put wooden stick into backpack (to keep for a later date) and went to another area of the shed to check for items
7 - After finding nothing i returned to leave and spotted a crafted splint on the floor - i guessed it had created the wooden splint after all but had been on a delay but check backpack and wooden stick was still there.

8 - In the spirit of nothing more than to expose and confirm a possible crafting bug i repeated steps 2 & 3.. exactly the same result ocurred, the splint was crafted, a rag was destroyed to make it, but not the wooden stick.

9 - I decided to repeat steps 2 & 3 a third time and yes, same result

Summarised findings: (TL;DR version)

When a wooden stick and a rag of "ruined" quality is used to craft a wooden splint, the rag is consumed in the process but not the wooden stick.

Sorry i didn't take screens, it didn't occur to me at the time