std::make_pair will make a copy. It will never return a pair with reference types.
Your return std::make_pair(true, myMap.at(...)); will return a std::pair<bool, MyData> (copying the result of myMap.at(...)), which converts to a std::pair<bool, const MyData&>, where the returned value’s .second is bound to the temporary that is immediately destroyed. This means your function returns a dangling reference.
Use std::pair‘s constructor:
// One of:
return { true, myMap.at(...) };
return std::pair<bool, const MyData&>(true, myMap.at(...));
(Also consider using just const MyData*, returning nullptr if it is not found. Barring that, std::optional<std::reference_wrapper<const MyData>> is also a good choice to replace pair with bool)
