FHA 203(k) Problem (So CA)

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Not sure if this is the right forum for this so please let me know if my questions are best answered in another forum!

So, I entered escrow about a month ago on a triplex that I purchased for 540k. The FHA consultant has said we need 100K in rehab. Contractor came in and said no problem. So our loan amount is 640K. Loan went into underwriting earlier this week. The underwriter has declined the loan. Underwriter said that on an FHA 203(k), the projected rents have to cover 75% of the monthly mortgage. Mortgage would be $4300 with projected rents calculated by the appraiser to be at $4800. Our lender said that our only option is to exit escrow or come up with 100K to bring our monthly mortgage payment within the guideline.

What am I missing?

Lender said that the 75% is the FHA guideline. I am a lawyer and have used all of my resources to find this 75% requirement to no avail. Why is that I can buy a SFH at 640k no problem, but I cant buy a triplex even though I have been approved to make the mortgage payment without even considering the supplemental rent if I had to?