Generally, buffer solution do not change pH after dilution (frankly saing, the change of pH cna be neglected, but exists). Multicomponent biochemical solutions contain many reagents, some of then can change pH indeed.
EDTA is used as disodium salt and is slightly alkaline, SDS (sodium dodecyl sulfate) is nearly neutral. So after mixing, the solution may show pH not exactly equal to needed value, but usually close to it. Then biochemists adjust pH (under control of pH-meter) to needed value,
usually by addition of small volumes of NaOH or HCl solution. This is absolutely normal procedure, especially for bulk solution.
I your case, when the final volume is quite small, you should believe in the pH value, or use pH-stat system.