Given that an autoclave system is also pressurized hobglobin suggestion is quite likely correct. The high pressure would drive the carbon dioxide to dissolve into yourbuffer, and you will obtain a sodium hydrogen carbonate solution.
I would suggest that you use a buffer system and filter sterilize your solution.
As for what buffer to you, I am not sure what you will be using the solution for, but there are several options
The table above gives CABS (4-(Cyclohexylamino)-1-butanesulfonic acid ) as an option.
You could also use an amino acid as a buffer.
Lysine, Proline, Cysteine and Tyrosine are within the 10.5 to 11 range.
May your PCR products be long, your protocols short and your boss on holiday