So, you agree, when C' and M are co-located, observers at rest in frame Σ claim the flash is located at Σ frame space-time coordinates of $$(d'/\gamma,0,0,\frac{d'}{c\gamma})$$. OK that is fine. Now, as any high school person would know, to determine the corresponding space-time coordinate in the Σ' frame, one applies LT. This is where you are having difficulty. Don't forget to apply LT.
Now, when you apply LT, you get the Σ' light flash space-time coordinate of$$(d'(1-v/c),0,0,d'(1-v/c)/c)$$ when M and C' are co-located.
From the Σ' frame, you agree the flash is at $$(d',0,0,d'/c)$$ when C' and M are co-located. Again, apply LT and you get the Σ frame light flash coordinate of $$(d'\gamma(1+v/c),0,0,d'\gamma(1+v/c)/c)$$ when M and C' are co-located.
As we can clearly see, when C' and M are co-located, SR claims 2 different light flash positions in the Σ frame and also SR claims 2 different light flash positions in the Σ' frame.