Strong gravity effects on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"> <mml:msup> <mml:mi mathvariant="script">R</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:math> -corrected single scalar field inflation and compatibility with the ACT data