Assuming that by "decontaminating every surface and item in contact with the electrode/buffer solution" you mean soaking the electrochemical cell in piranha solution, there should be no fault with either the solutions, cell or working electrode.

What about the other electrodes? Did you flame-clean the Pt counter electrode? Did you exchange the Lugin capillary / frit / etc. separating your reference electrode from the main body of the solution?

Frankly, I would suspect the counter electrode as the most likely culprit, particularly so if it is in a plastic sleeve that does not encase it perfectly (i.e. allows solution to enter the area between the counter electrode and the inner surface of the sleeve).

Methylene Blue can undergo electropolymerization - most likely

you will have a MB film deposited on your working electrode.

As pointed out by Tomasz, Piranha etch will remove it.

Hi, i would suggest you to change the working elctrode wit Ti electrode instead of gold disk electrode.

You should do the etching for Ti plates in 10 % oxlaic acid solution before using it.

There can be additional tricks which depend on construction of your cell and electrodes. I mean some possible "dead zones". E.g. if your gold disk electrode pressed (sunk) in an insulating plastic the depolarizer (in your case MB) can go in-between walls of metal and this plastic even despite strong contact due to capillary forces. It depends. Other issue is SCE construction i.e. its membrane which also can absorb MB and further works like depot. I would recommend to use other electrodes which were not in contact with MB solutions. May be this can help.

There can be also other "dead zones".

The recommendations are appreciated, thanks y'all. I'll try washing everything in piranha solution again and maybe I'll try and diffuse methylene blue out of the propesed capillary like openings between the gold disk and sleeving by incubating the electrodes in distilled water for a couple of days.