*** Topic moved from Process & Procedures to Progress Reports ***
From my years of being on this forum, this is just not the case. Very rarely do they say the exact reason, especially when it comes to bonafides of the relationship. They keep it extremely vague.
Unfortunately, case being returned means the case has been denied. There are no appeals available for K1 visas. HCMC is a very tough consulate, I’ve read many denials posts specifically for HCMC over the years on this forum. You need to spend significant amount of time together and I recommend going the CR1 route instead of you plan to refile.