mic5504 + esp8266 = going mad
What you describe is typical behaviour for those LDO regulators (like the MIC5504) which require specific input &amp; output capacitors, and those capacitors are either:


not fitted; or
are fitted, but they are too far from the regulator; or
are fitted, but they are the wrong type of capacitor.


Here is the relevant section from the MIC5504 datasheet, with some important points highlighted:







With missing / incorrect capacitors, especially the output capacitor, the regulator can oscillate causing incorrect operation of the connected device(s) and an abnormal reading on a multimeter (if you used an oscilloscope to look at the output voltage, then the output oscillation becomes visible).

You got different behaviour when you were missing the decoupling capacitors for your flash memory, as they are also effectively across the regulator's output and therefore altered the problem.

I've asked you to add a schematic and photos into the question, in order to see your specific implementation of those requirements. If those updates lead to any further analysis, I'll add it here.