If the FA have spotted a potential ineligibility occurrence and then contacted the club in question for further details and to make an attempt to get to the bottom of it - if after all of that they came to the conclusion that the ineligible fielding had indeed occurred and a charge was therefore warranted, they must have been certain that their conclusion was correct. If they weren't certain they either continue discussions further or they just drop the whole thing right there due to insufficient evidence.
To issue a charge after five weeks of investigation but then turn around just days later and say there wasn't a problem after all is very odd.
To issue a charge after five weeks of investigation but then turn around just days later and say there wasn't a problem after all is very odd.