Your original question was about liability and how could this have been missed by the appraiser (since you are posting the question on an appraiser forum).
I cannot tell you how it could have been missed.
But as far as liability goes, as others have pointed out and in regard to the appraiser, it is doubtful the appraiser has any liability to you. The appraiser was engaged by a lender for purposes of evaluating the property for a mortgage finance transaction. If there is harm, it is with the lender. Certainly, for any lender who has been replaced by means of refinancing, there is no harm to them. The last lender who is holding the bag has been harmed. Since FHA is also an intended user, the argument is they've been harmed as well.
But the question of you being not being harmed due to the original purchase-transaction appraisal is especially true if, at the time of your original purchase, you made your offer without the appraisal: even though you are not an intended user for the appraiser, you made your offer before an mortgage finance appraisal was completed. So, it was worth what it was to you. There are court cases that set precedence for what I am stating.
Now, here is my advice going forward:
A. Get the engineer to evaluate the property. Find out what needs to be done and get bids on how much it will cost. You should do this whether you make the changes or not; if nothing more than for peace of mind.
B. If you want to get financing at the lowest rates, then complete the modifications that the engineer says must be done and get the final engineering certificate.
C. Contact an attorney and see who, in your original purchase transaction, may be liable for any damages based on what you have discovered as of recently. There may be some liability, but there may not be. An attorney, well versed in real estate and contract law, would be the one who can address this issue.
It is an unfortunate situation; and, yes, that is easy for me to say when you may be out thousands of dollars to make whatever modifications are needed. I'm not dismissing or diminishing that. But the practical advise is as I've outlined above.
Best of luck to you!