You didn't mention whether you MLF'd originally, or tested for completion of MLF... if not, I'd agree with the probable spontaneous MLF being a good possibility here.
There's a useful sulfite calculator that I use here:
winemakermag.com/guide/sulfite
According to that, for a red wine at pH 3.6 going into a 225L Bordeaux barrel, you'd be looking to add 19.7g sulfite.