I had the same experience, I cannot restart a timer.
Cancel and re-start is not an option, that will complicate automations.
My use case is a count down timer to turn off a light, every time there’s motion reset the timer. This way I don’t have lingering and overlapping automations.
I would have (2) Automations and one timer per light:
(1) Turn on the light and start (or restart) the timer on motion
(2) automation to turn off the light when the timer hits zero
The timer would vary based on the use of the room, living room set higher (10m) because you sit idle, working area of a kitchen lower (2min), because you’re constantly moving.