I have heard of this problem with specific ESP8266 modules. The flash memory chip is separate from the ESP8266 microcontroller chip so the manufacturers of these modules are free to use any flash chips they like. Unfortunately it’s impossible to see which flash chip is used on the modules that have the metal shield. Even on my ESP-01 modules that don’t have the shield, I found very little information from a search of the model number on the flash chip. Then of course you can’t really trust the labels on these Chinese chips anyway since that is easily faked. It’s really a nightmare but you can definitely get ESP8266 modules that have good quality flash chips, or even replace the garbage flash chips if you’re skilled at soldering.