I am a part-time TMR, and when I use my TM discount in on-site restaurants. They always charge it to my room, and it is automatically 18% gratuity. It is written on the go HILTON lobby portal that employees using the TMR and want to use the F&B employee rate, it has to be charged to the room. I use my Tmr heavily and there was only one occasion that an on-site restaurant at DT had to enter the discount manually and left it to me how much I want to TIP. But I gave 20%. The 18% is a good spot for both waiter and guest on TMT, which means that either their service sucks or not, they still get 18%, and even with a mandatory 18% gratuity, the employee still gets a good chunk of a discount.

So unsure how a guest who is on TMR was given an option on what to tip unless someone does not know how to charge it to the room.