My tap tempo used to work good for 2 years after I bought the Helix floor LT. After Firmware update 2.92 appr. a year ago it started to have the problems mentioned here in this thread. I bet, it is a software issue.
Sometimes it sets the tempo to very high already after the first tap. The only thing, what helps, is to step on the FS very light and quick. I think, Helix interprets a heavy long tap as two beats.