Washing soda can be anything from anhydrous to decahydrate.
Mix up a test batch of developer with the right amount of carbonate and see how that goes.
(quickly nicked from here)
Your developer should be at around 10.25 - 10.30 IIRC. Shifting the buffer to the right by adding an excess of carbonate will shift the pH point upwards. This will make the developer overactive and it will affect color balance as deeper layers (i.e. red, green) will develop faster in relation to the top layer (blue) than intended. It will also affect b+f, but the degree to which it happened here still makes me wonder whether this really explains what you're seeing. Still, it's definitely worth trying.
Also, if you can, test your developer with ECN2 film instead of C41, although the base of your C41 film processed properly in ECN2 should come out reasonably close in terms of density (not necessarily hue!) to the C41-processed sample you've got.
Mix up a test batch of developer with the right amount of carbonate and see how that goes.
pH will end up way too high:
(quickly nicked from here)
Your developer should be at around 10.25 - 10.30 IIRC. Shifting the buffer to the right by adding an excess of carbonate will shift the pH point upwards. This will make the developer overactive and it will affect color balance as deeper layers (i.e. red, green) will develop faster in relation to the top layer (blue) than intended. It will also affect b+f, but the degree to which it happened here still makes me wonder whether this really explains what you're seeing. Still, it's definitely worth trying.
Also, if you can, test your developer with ECN2 film instead of C41, although the base of your C41 film processed properly in ECN2 should come out reasonably close in terms of density (not necessarily hue!) to the C41-processed sample you've got.