I do remember it, Bob Dole and Phil Graham (who had his own flat tax plan) both attacked him on it, and Bush trying to indirectly attack him on it, by offering his own tax plan in 2000 (McCain did attack Forbes on it though).
HOwever, that said, Grahams attacks on Forbes was simply 2 guys attacking each other over different flat tax plans, they agreed in principle (though it was really really really strange how vicious they were with each others plans).
Gramm might have been a good president but he was a TERRIBLE candidate. As you say, I don’t remember much animus on their conception of a flat tax, only fine point discussion.
Dole only went with the flat tax when he was desperate at the end, as I recall, so it wasn’t so much that he was anti-Forbes as he took up the mantle late in the game.
But you’re right—having been reminded, now I DO recall Bush bashing him on it. I still forget what Bush had to complain about, but I remember he had something he disliked about it and he smacked Forbes on it at some point.