Keep an eye out for fake MAX31855 thermocouple sensor modules!
Unfortunately, as pointed out on this forum post, there are plenty of fakes on the market. Look out for the following marksings on the chip:
The modules have different white logos on them, depending on the distributer. From AliExpress - CJMCU, and from eBay - QiFEi.
They absolutely don't work, and only produce junk values. Hopefully you read this in time to throw them away and buy new ones before you lose your mind in frustration.