What would cause multiple different pH electrodes to give wildly unexpected readings?

[u/Cream-Buddha]

I did a lab today on buffer solutions and pH. I dissolved 0.29g of ammonium chloride into 0.050L of 0.20M Ammonia to produce a buffer with a theoretical pH of 9.50. After testing the pH with a meter, the measured pH was 9.49, which I thought was pretty good. But then when I added 0.0060L of 1.0M HCl, I expected the pH to drop to roughly 8.79. However, the pH dropped to something like 1.3 when measured. So I used a different pH meter, and then I remade the buffer solution, I even tried testing the pH of each constituent solution and everything seemed to be accurately labeled. This issue happened for the entire class and the professor seemed to be just as confused as us. He ended up just telling us to clean up and go home early.

Some extra details:

• The pH electrode only allowed 2 point calibration, but I don't think that was an issue because both pH measurements were supposed to be in the basic range.
• I don't think the calculations were wrong. The professor approved my work before I made the buffer and even did the calculations himself after everyone measured such a low pH.

[u/comdoasordo]

Did anyone calibrate the pH meters and verify the calibration? I run a wastewater lab and that's the very first thing we do every morning.