The path you send is treated as a specific filename unless you include a wildcard character.
Try:
FString FinalPath = FullFilePath / TEXT("*");
FileManager.FindFiles(Folders, *FinalPath, false, true);
Also, I haven’t tested this, but the documentation for FindFiles says “files or directories” rather than “files and/or directories” so you may not be able to pass “true” to both. I’d be a bit surprised by that, but if you still have trouble it’s worth a shot just doing one or the other.