We had this occur several years ago where the rate plus the margin was lower than our floor. We showed the "floor" rate in the example with a footnote indicating that the rate shown in the table was the lifetime floor.
Year: 19XX Index: 6.000 Margin: 1.500 Annual Percentage Rate: 7.900(1)
Footnote (1) This rate reflects a lifetime floor of 7.9%
The Minimum Monthly Payment was based on the 7.9% APR rather than the index plus margin. Using the index plus the margin would have resulted in understating the minimum payment. I don't know if this is the correct way to show this. But this disclosure was being used prior to my coming here two years ago and has been through at least one OTS exam that I know of since then. It was not questioned or mentioned by the examiners at our last exam. Hope this helps.